Joker is nihilism incarnate and Batman's greatest weapon isn't his gadgets, or his money, or his plot armor, it's his empathy and his belief that everyone deserves a shot at redemption.

He's a man who wants to prevent the suffering of others because if he couldn't save his parents, then he can at least try and save everyone else.

Bruce will save anyone no matter how heinous their actions are, because he's a flawed, but well-intended man who believes everyone deserves a shot at redemption.

Joker kills anyone and everyone just for a laugh. He's a sadistic, narcissistic mass murderer who lacks any empathy for anyone and wants to see the world suffer the way he's suffered.

It's the duality that makes them interesting. And the fact that Joker is right in the sense that one bad day made the two of them who they are. The duality between them is one of the most interesting things I've ever seen (when it's written well, that is).

Okay here's a long one...

I get that Joker is used a LOT. But people who want to see him gone for good are so funny to me because they clearly don't understand why Joker is so important to the Batman mythos and why he's Batman's arch nemesis.