1. Light from Death Note
Young Light Yagami only wanted to help the world when he found the Death Note, a magical notebook that would kill anyone whose name he wrote in it, in the manner he described it.
He used it to kill criminals who escaped justice or were in the middle of their schemes… and then he just started killing all the worst criminals, even if they were already in jail.
And then he killed a detective who had stumbled onto his secret — in order, he told himself, to serve the greater good.
That was a first step on a slippery slope to eventually killing everyone he decided was a criminal, regardless of what the law said, as well as anyone who tried to stop him. By the time he was eventually caught, he had murdered thousands.
2. Father from Fullmetal Alchemist: Brotherhood
The antagonist of the original Fullmetal Alchemist manga and the second anime Brotherhood bears a striking resemblance to protagonists Ed and Alphonse Elric’s father Van Hohenheim, but he’s quite different.
Mainly in that he is willing to kill the citizens of whole cities and even entire countries in order to advance his own power, and that he succeeds in both, killing millions.
The reason why he does the former is to free himself from the flask he was trapped in, which is kind of understandable, but the country sacrifice was simply to absorb the power of God, which is a feat as arrogant as it is evil, and it's pretty evil.
Oh, and he’s more than willing to kill his own children, the Homunculi, in order to achieve his ends. However, he’s only one of FMA’s many horrible fathers, as you’ll see....
3. Johan Liebert from Monster
If you’re the bad guy in a manga series titled Monster, chances are you're going to be something special. And Johan Liebert is — he’s a serial killer that would make Hannibal Lector step back and say “whoa.”
He killed his adoptive family, he killed the co-workers of the surgeon who saved his life, he even managed to talk all the students and instructors of a secret Germany military training program into killing each other. And then, after the surgery, he really got busy murdering people.
He was actually a leader of sorts for Europe’s serial killers, who did his bidding even as he occasionally killed them (which, to be fair, kind of goes with the job description).
He’s actually so evil and murderous that a Neo-Nazi party tried to get him to become the new Adolf Hitler — which wassn’t Johan’s style so he massacred most of them so they’d leave him alone. When people are asking you to be Hitler 2,0, you know you’re a bad dude.