One of my favorite Batman comics is The Killing Joke. Alan Moore created a classic "what-if" Joker story. The ending features Batman and Joker laughing in a sort of understanding for once.

"All it takes is one bad day.."

Batman did experience his own "bad day," the day he lost his parents. Unlike the Joker, he goes by the book and tries to make things right. The Joker, in this depiction, uses madness as a shield to hide away from his pain and cause chaos.

Bruce is at the very edge of crossing over to madness, but he chooses to hold himself back. For one, he swore to provide justice to Gotham and Batman does NOT kill.

One example of him finally losing it is in another favorite comic - Frank Miller's The Dark Knight Returns.

He played into the Joker's hand and actually killed him. The Joker finally won (as is evident by his devilish grin even in death) their constant war and pushed him over the edge. To become just like him and finally take him out.

It's always been a head canon of mine that Joker always wanted things to end this way. He has constantly provoked more and more violent behavior from Bats. Death was the prize all along.

Batman even fought against an ally. Bruce really ended up hating Clark and became more cold and bitter.

He became something that went against his own morals.

Batman should forever be the light that shines in darkness.