A hero who's gradually turned "dark"

[BWTraveller]

I've seen a lot of series lately where the hero very quickly goes "dark". In no time, he abandons his humanity and becomes a cold, calculating, cruel creature that hates everyone around him. Off the top of my head there's one of these which I like (which has an anime in the works), in which one of four heroes is tricked, framed for a crime he didn't commit, and rejected by the entire world as well as his supposed comrades; he becomes cold and ruthless, although he still remains for some time quite weak. Others I know of have the hero become even darker even faster, often with at least some clear changes to appearance. I'm OK with the concept of a hero going dark, but I'd like to see something more gradual, where you can really see the hero struggling against this side he knows well enough to resist, and see just what makes him slowly lose to this dark side.

This doesn't have to really mean that the hero just changes; it's fine if there's some sort of separate force or entity that's trying to take over. For instance, the best example I can think of that I'm currently watching is Yu-Gi-Oh Arc-V, in which the hero is actually a part of a superbeing that wants nothing more than to rage out against the universe that constantly demanded blood. Bit by bit, as he joins with other parts of this being and is forced through all sorts of trials that wear at his ideals and call forth his anger, he starts to lose control and allow this second self to take over from time to time. He undergoes spells of darkness rather than just gradually sliding down the slope, but the effect is similar.

There are plenty of ways this could work, from a villain trying to break there being something inside the hero as mentioned above or the hero just having issues he needs to deal with, and it could be just him slowly becoming more dark, violent, cruel, etc. or an increasingly frequent swing from hero to villain. Just as long as the core remains the same, a hero who slowly develops a truly dark side and struggles to fight against it. Bonus if this dark side is a legitimate villain or even the ultimate villain, leaving the question of whether the hero will fall and his comrades will have to destroy him or he'll actually manage to defeat this dark side himself.

[AnimeFangirl]

Berserk? Been a while since I read it, but the hero had a suit of cursed armor that was trying to take him over. He also goes through a lot of unpleasant stuff in the series that threatens to drive him mad.

[cyberdemon]

There's Shinn from Gundam Seed Destiny. He wants to do what is right but he is also very arrogant and bitter due to personal tragedy. He gets manipulated by those around him and ends up being the dragon to a man who doesn't mind wiping out countries that don't want to follow his plan.

[LKK]

Have you tried Chain Chronicle? Darkness of the soul is much like a plague and anyone of the Light can be infected including the hero that the people are hoping will save them. (CR has the Japanese language & Funimation has the English.)

[kitten320]

Yomi from Ga-Rei Zero. Starts as a loving and cheerful character but because of circumstances gradually goes to the dark side until she gets possessed.

Zwei from Phantom: Requiem for the Phantom doesn't exactly go bad but he does eventually become the most feared assassin who has no remorse once he is in assassin mode.

Kurapika from Hunter x Hunter. He doesn't go bad per say but he does become far more gloomy and self observed as series progress. His life goal gradually eats him from the inside making him less caring.

Re:Zero kara Hajimeru Isekai Seikatsu - MC does not exactly go bad but he is pushed into madness where he stops caring but does manage to recover his old self back.