While anime often chooses to highlight villains who redeem themselves and join the good guys, there are some shows that are all about anime good guys who break bad.
In some cases, characters start off as genuinely good people, but circumstances tempt or force them into a life of villainy. In other cases, characters are actually evil all along, but their sinister motivations are not revealed to the viewer until the other characters learn the truth. In one case, a character tries desperately to make himself good, but ultimately gives in to his innate villainy. No matter the circumstances, it's fascinating to watch someone who seemed to be morally just give in and embrace the dark side.
At the start of Death Note, Light Yagami is an ordinary teenage boy with a strong sense of justice. When he picks up a magical notebook that lets him kill anyone whose name he writes down, he's initially reluctant to use it. However, he quickly decides to put his newfound power to use, and starts taking out criminals. His motives are pure, if dangerously misguided, and he genuinely believes that he's making the world a better place.
Soon, Light begins to do things that are inarguably unjust. He starts killing people who have done nothing wrong, just because they stand in his way. He even considers letting his own sister die to avoid getting caught. The Light Yagami from the beginning of the series would be disgusted by the person he becomes.
Griffith of Berserk is so beautiful (and so evil) that he's literally supernatural. While he starts off as an honorable — if somewhat arrogant — person, he changes quite a bit over the course of his life.
At first, Guts and Griffith are partners. It's not entirely equal, since Guts was forced to become a mercenary by Griffith after losing a duel, but Griffith considers it a partnership nonetheless. When Guts eventually wins a duel that earns him his freedom, Griffith is furious, and starts pursuing his true goal, which is to build and rule over his own kingdom. In order to accomplish this, he joins up with a powerful group called the God Hand, and uses an eclipse to kill every one of his followers in order to fuel his own transformation into the powerful Femto.
Despite all this, Griffith does truly want a normal life, so watching Guts and Casca love each other fills him with jealous rage. What he actually wants is to marry Casca and have children with her.
Ken Kaneki of Tokyo Ghoul starts off as a mild mannered young literature student who's drawn into something grotesque. He's attacked by a ghoul (an anthropomorphic being that eats human flesh), then turns into one after he accidentally receives ghoul organs in a transplant. His giving into his newfound desire to eat human flesh doesn't make him villainous; he finds a way to do so ethically after he meets the members of Anteiku. It's only after he's brutally tortured that he loses his desire to be kind. When this happens, he becomes willing to act ruthlessly towards others if it means furthering his own goals.
Sōsuke Aizen of Bleach is secretly a villain from the start, but most viewers don't know this until a major plot twist occurs. At first, he seems like a nice guy who is totally dedicated to Soul Society. He's a bit of a stickler about rules, but acting lawfully doesn't make someone a villain. As it turns out, his whole persona was a front meant to disguise his true intention, which is to crush Soul Society. His plan is set in motion when he fakes his own death, implicating Soul Society's newest members (Ichigo and his friends) as potential murder suspects in the process.