Rules: They have to be one being, not an organization or an idea (like WICKED from Maze Runner). They cannot be evil for the sake of evil- I’m looking at you, Leck. They have to prove their evilness, so it can’t be just because I don’t like them. They must be from books, at least originally. Oh, and yes, there will be spoilers.

And so, in no particular order, I present:
Bernd Siegert- This guy is from Ursula Archer’s murder mystery Five. Not necessarily YA, but awesome nonetheless. Siegert is a twisted, twisted man, although really, you can’t help but pity him. He lost his entire family (wife, daughter, and sons) in a fire, and for years thought it was his fault. Years. But then he finds out that the deaths were caused not by him, but by a fairly reckless and terrible group of geo-cachers. I know, right? One volatile man, three bystanders, one outraged woman, and the Siegert family minus Bernd, who was working. I won’t go into the details of how his family died, but let’s just say it messed poor Bernd up for good.
When he finds out about the involvement of the five geo-cachers (hence the name Five), he decides that a little retribution is needed. Body number one is a woman he shoved from a cliff. Body number two is actually a body part- the detectives chasing him just find a hand. Then a pair of ears. Then a mutilated corpse. Eventually, it gets to the point where he’s killed all of his targets but one, who is now severely mentally damaged. He also plays a very good game of cat-and-mouse with the cops. He has them find all the bodies through geo-caching, which is some poetic justice for his family’s murderers. And yet despite him slaughtering all of those people- absolutely brutally- you can still feel sorry for the man.

Bonus Points: I'm pretty sure mutilating someone and chopping off various body parts for the police to find is beyond evil.

Daniel Redding- From Jennifer Lynn Barnes’s Killer Instinct, here is… yet another serial killer. But an exciting one, so it’s totally worth it. Before his imprisonment, he had one very simple method of killing: Bind them. Cut them. Brand them. Kill them. And now, a copycat is out emulating him, and the only person with any insight on the copycat is Redding, who has one simple demand- he wants to see his son. Not unreasonable, but considering he made his son watch the women he tortured, you can imagine that unfriendly conversation ensues. This guy has some dark humor, too: he quotes Shakespeare, does everything to bait the FBI, and often brings up the women he murdered just to watch their reactions.
But even better than his witty repartee is his plan. I love villains with a brilliant plan, and Redding comes out near the top with his. So he can’t murder anyone anymore? That’s fine. He can send out another to do it for him. Here’s a conversation between him and the protagonist, Cassie:
“Is your apprentice a college student?” I asked.
Redding didn’t hesitate, not even for a second. “Yes.”
“Is your apprentice someone who’s never been to college?”
If Redding thought it odd that I was asking two versions of the same question, he gave no indication of it. “Yes.”
“Is your apprentice under the age of twenty-one?”
“Yes.”
“Is your apprentice over the age of twenty-one?”
He smiled. “Yes.”
“Is your apprentice someone you met through the mail?”
“Yes.”
“Is your apprentice someone you met in person?”
“Yes.”
Think he was lying? He wasn’t.

Bonus Points: Making his own son watch him torture and kill his victims.

Nero au Augustus- I’m quite honestly surprised that not many people have read Pierce Brown’s Red Rising. In my opinion, it trumps Hunger Games with ease. Especially with its antagonists.
Nero au Augustus- the name sounds like it belongs to someone who is pretentious, arrogant, and rich. Congratulations. He is. But he’s also cunning, cruel, ambitious, and dangerous. Augustus rules the planet Mars, and in his position of ArchGovernor, has all the wealth and power he needs. He has only two heirs- Adrius (the Jackal) and Virginia (Mustang), so to combat the other Houses, who definitely have a sort of coup d’état in mind, he collects the finest of Mars’s Institute (which is similar to Battle School in Ender’s Game). He uses them to take out rivals, seize power, and just add glory to his name.
Despite his greed, Augustus wants more than the throne of the Sovereign, the ruler of the solar system. He wants to do whatever it takes to keep humanity from fading into a thing of the past, and he will do anything to make sure he reaches his goals, even if his methods are beyond questionable.

Bonus Points: Having the most conditional love I have ever seen in a parent.

Valentine Morgenstern- I know, I know, Mortal Instruments has some problems. I’m not much for the romance in it either, and to be honest, I didn’t like Sebastian as a villain at all, so books four through six were kind of lackluster. But to me, Valentine is like a better Voldemort. Voldemort just wasn’t relatable, and definitely had that ‘evil-just-because’ thing going. But back to it.
The best thing about Valentine is that, in a way, he’s right. Let’s be blunt here, the Clave kind of sucks. No, you can’t try to save people’s lives. Leave that to us in our total incompetence. Seriously, their Inquisitors are terrible, there are a fair amount of traitors, half of their youth joined the Circle at one point, and all of their new generation basically says, “Forget your rules!” and do whatever they want. So Valentine exploits it again and again and again, whether it’s using Hodge, who the Clave seems to be okay with, manipulating the Inquisitor (why did they hire her in the first place; she’s a psycho!), or using the many traitors that he planted in their midst, including their own Consul. For real, Clave, where are your background checks? Valentine points it out multiple times: the sheer inefficiency of the Clave, and the way that its own members practically have to make pariahs of themselves to get anything done.
The other thing about him is that he’s a zealot. He firmly believes in his plan, can easily justify it, and won’t waver for anything- not for his wife, not for his son, not for his adopted son, not for his daughter, and not because he’s facing an international demon-hunting government that basically wants his head on a silver platter. He will also go to any lengths to achieve his goals, even if it means working with the enemy.
And despite all this evil you can see in him, you can also see the humanity in his love for Jocelyn and Jace, his unwavering belief that he is a patriot, and his emotions after he (kind of) kills his adopted son and then gets murdered by an angel. Fun guy.

Bonus Points: Being dedicated enough to kill one of the few people he cared about for the cause.

Dolores Umbridge- Not a single person likes her. Not one. She’s even more hated than Voldemort, and her penchant for kittens and frills makes you want to Avada Kedavra the woman even more than you want to kill Draco and Cormac McLaggen (hated him too). From the frills to the pink cardigans to the hem hem, this lady has absolutely zero redeeming qualities, yet doesn’t suffer from that ‘evil-just-because’ thing I keep mentioning (hi, Voldy).
But seriously, I hated her right from the beginning. But it sounded for a teensy moment as though you were suggesting that the Ministry of Magic had ordered an attack on this boy! Yes, that’s exactly what he’s saying, okay? No need to clarify that one; Dumbledore made it pretty obvious. And then all the ‘no, no, Voldemort doesn’t exist anymore; Harry’s hallucinating’ thing, followed by practically slicing his hand open every night? I think Hermione said it best when she called Umbridge a hag. That’s kind of the only word for her.
All this made the end for Umbridge so satisfying. In Order of the Phoenix, she got abducted by centaurs for calling them ‘half-breeds’, which is probably the only time I ever rooted for the centaurs. And then at the end of the whole series, she got arrested for crimes against Muggle-borns and locked up in Azkaban. J.K. Rowling delivers the harsh justice for this terribly irritating and totally evil woman.
Bonus Points: Forcing students to slice their own hands open while surrounded by kittens and bows.

The Darkling- Although he was stuck in the rather lifeless Grisha trilogy, this is a seriously awesome villain right here, mostly because of the question, Is he actually the villain? He’s powerful, ancient, cunning, ambitious, and utterly ruthless, which makes it fun to watch all of the things he does.
On a small side note, I hated the protagonist of this series. Alina is dull and whiny, with absolutely no sense of loyalty and to be honest, just no sense in general. But this guy made the series worth reading, even if the ‘darkness never dies’ phrase from the second book was totally incorrect. When Alina throws her hissy fits on how life isn’t fair to her, the Darkling calls her out on it: “Still she talks of fairness. What does fairness have to do with any of this? The people curse my name and pray for you, but you're the one who was ready to abandon them. I'm the one who will give them power over their enemies. I'm the one who will free them from the tyranny of the king.”
Alina tries to make him seem completely evil again and again and again- not that he isn’t a murderer and an all-around merciless guy- yet, she just can’t do it because the Darkling isn’t entirely wrong. His methods are a bit… overboard, but the King and Queen are terrible rulers and Alina is pretty selfish. He really would do a better job ruling, and he has his sort of dark charisma (pun not intended), so you just can’t help but root for him, even if his end was entirely unsatisfying.
He has more than just charisma, though: his plans (with the exception of the one at the beginning of Siege and Storm) are well thought-out and multi-dimensional, and his complete understanding of human behavior also gives him an edge. I’m not going to be one of the desperate fans who begs to revive this character, because even the best villains can turn irritating if they’re just sort of there for too long, but I will say that the Darkling is one of my favorite villains of all time.

Bonus Points: Destroying an entire village to prove a point.

Mayor Prentiss- Another underappreciated series: Chaos Walking. Prentiss is a pretty great antagonist, no doubt about it, because he’s so complex. He can be ruthless (burning down towns, murdering all the indigenous) but merciful (spares the lives of New Prentisstown). He truly cares about Todd, but is indifferent to everyone else, even his biological son Davy. He flickers between good and evil, and one of the main questions of the last book, Monsters of Men, is whether Prentiss was redeemable.
Given his actions in the first two books, which include burning down Farbranch, branding and enslaving the indigenous Spackle (yes, that’s what they’re called), branding all the women, slaughtering the Spackle, waterboarding and executing innocent people, murdering one of the protagonist’s foster fathers, and murdering his own son, it seems impossible that this man is redeemable. But he manages to get almost everyone on his side rather than his rival, Mistress Coyle, despite being known as the despot who murders a ton of people.
You can see some of the better side of him when he’s with the protagonist, Todd. He teaches him to read, goes to great lengths to save his life, treats him as his son (even though he shot his actual son), and even joins Todd’s cause, bringing his power and influence into the equation. If Todd’s foster father hadn’t survived, would the Mayor have gotten his redemption? He might have.

Bonus Points: Very nearly redeeming himself in Todd's eyes.

Slagar- Redwall usually doesn’t have good villains, because they’re all ‘evil-because-I-feel-like-it’ and are basically just cruel animals with a horde/army/gang behind them. The only exceptions are Slagar, Sawney Rath, Swartt (kind of), the Marlfoxes, Pitru, and Zwilt, which for a twenty-plus book series… Anyway. Slagar is unique from the rest of these sadistic little woodland beasts because he doesn’t rely solely on fighting skill or vicious horde. He has a brain and uses it, too.
His gang is only fifteen members at most, yet he manages to steal a load of children from what’s basically a fortress with a fighter channeling the spirit of the uber-warrior. The gang members are all terrified of him, which is understandable: he carries a bolas, a weapon with three heavy stones attached to a handle by rope, and he knows how to use it. Yet he manages to manipulate them all quite easily- it’s mentioned that he’s run his slaving scheme multiple times, all with different gangs, because he tricks the other members into killing each other off. He’s a heartless little fox, and a pretty good villain for being in a kid's book.

Bonus Points: Leaving his enemies trapped, suffocating to death, instead of killing them outright. And stealing their children to force them into slavery.

Alexander Vosch- All right, it wasn’t much of a secret that this guy was evil. When you just straight-up murder the protagonist’s dad in the dirt, it becomes pretty clear that you’re not the good guy. But the fun part about Vosch isn’t the fact that you didn’t see him coming. It wasn’t hard to guess. The fun part is that you have no idea what he is or what he wants.
It’s revealed that Vosch isn’t actually an Other. I know. Mind blown. So what is he, if he’s not alien but has a load of futuristic technology? I have my theories… But I still don’t know what he wants. For some reason, it’s important that some people survive the Waves, but his purpose isn’t clear. He gives Ringer the 12th System, knowing full well that she’ll betray him the second she can, but doesn’t even hesitate to do it. He has all the memories of any person who passed through Wonderland, allowing him to predict the actions of almost anyone, but what does he want with the memories? Is that all?
I have so many questions about him that I have to applaud the author for writing such an intricate antagonist.

Bonus Points: Playing chess with captives. I mean, of all the things.

The Jackal- Adrius au Augustus, the son of Nero au Augustus, and supremely creepy villain known better as the Jackal. Oh, where to start with him? He’s a cannibal, a mass murderer, and a patricidal nutjob. He’s in his early 20’s when he first gains his power, and will use anyone and anything to get what he wants: first the approval of his father, then the throne of the Sovereign.
Apparently, he’s been willing to do anything from a very young age, because he hired someone to murder his older brother just to get his father’s attention. When he revealed to his father that he was indirectly responsible for his brother’s death, his father rejects him (as expected). So he murders his father, too. He’s also willing to do anything to himself. When the main character, Darrow, pins his hand to a table by sticking a knife through it, the Jackal saws it off. His real weapon isn’t his pistol, it’s his tongue. Like the aforementioned Darkling, Adrius has what Darrow describes as a ‘cold charisma’, which he uses to gain his followers- the most vicious, cunning young members of the Gold caste.
But just because he uses his tongue doesn’t mean he can’t use the weapons, either. He quite easily decides to fire two nuclear bombs into sectors full of people just to prove a point to his twin sister, Mustang, who has ranged herself against him for obvious reasons. His genius battles hers, and it’s exciting to see whose wits come out on top. So if you’re looking for a brilliant, shady, overall terrible guy, look no further. The Jackal- even his name suggests it.

Bonus Points: Slicing his own hand off to prove a point.

UPDATE: O'Brien- I have no idea what his first name is, but wow... he's more than a little messed up. To start with, 1984 is hardly a cheerful book, what with a government that controls literally everything. Seriously, this guy can read your mind. And he's downright happy about using what he finds against his... patients, for lack of a better word. He can find a worst fear and torture you with it, or he can find a happy memory and make you hate it. His job is to make people who hate the government learn to love it, and he's very persuasive. Very, very persuasive. So, if you're into dystopian fiction, you really can't get more dystopian than 1984 and you can't get much more disturbing than O'Brien.

Bonus Points: Being willing to allow rats to gnaw through someone's flesh.

Jackal- No, he's a vampire from The Immortal Rules. But much like the other Jackal on this list, this one is a complete nutjob. Evil warlord check: rules a sizeable domain? Check. Uses power for bad things? Check. Pointlessly cruel? Check. He takes pit fighting to a new level when he traps humans in with zombie-like Rabids and watches them die. And why, you ask? Just for fun.

Bonus Points: Staking his own 'sister' through the stomach.

Villains Who Don’t Live Up To The Hype
Leck- Not only did you not even meet the guy for three-quarters of the book, but he definitely suffers from that ‘evil-for-no-reason’ thing. Torturing animals and small children? Really? Not to mention that for a man who can control other people with his voice, his death was ridiculously stupid. Katsa didn’t help. I hated her too.
Voldemort- I realize that my opinion’s unpopular here, too, but come on. He’s definitely evil for no reason. Not to mention that his plans kind of stink. Harry lucks out again and again and again due to the sheer incompetence of Voldemort and his Death Eaters.
President Snow- Another evil for no reason. I will watch children fight to the death! Why? Because it entertains me! I also torture people for fun and kill off all my henchmen!
Sebastian Morgenstern- Not only was his ‘revival’ utter nonsense, but he is also evil for no reason (notice a trend?). This gets frustrating. There’s no redemption possible, and all of his plans are basically ‘How can I kill the most people and still get my sister to fall in love with me?’
Patch- Hush, Hush was a book I wanted to like. But this guy should definitely be the villain. I mean, he admits to trying to murder the protagonist multiple times, and she still falls in love with him? Come on.

Evil King of Adarlan- Don’t even know his name, or really, anything about him other than he’s evil. When I don’t even have a name for the man and he isn’t supposed to be a figure of shadows, then that’s just poor.