9. Killed Lex Luthor's Son - Infinite Crisis
Infinite Crisis is one of the biggest events in the DC universe, similar to Marvel's Infinity saga in the massive galactic scope. These crossovers bring every single hero, villain and galactic entity into one big battle for the fate of the multiverse. Every DC character that is, except for the Joker.
Infinite Crisis starts out as a massive plan executed by Lex Luthor's son, Alexander.
Being the only hero on Earth Three, the world where the Justice League is a crime syndicate, he had formed a plan following Crisis on Infinite Earths to recreate the multiverse in his image - as all Luthors are want to do.
But the young Luthor made several mistakes with his allies and enemies and was defeated. He went into hiding, where he encountered the Joker working with his father.
The pair were very cross that they had not been allowed to work with Luthor Jr, especially the Joker, who burned half of Luthor Jr's face-off with acid and then shot him in the head killing him.
It isn't exactly a master plan brought to fruition, or a loved one of Batman's finally biting the bullet, but the Joker inadvertently won. He could be seen as a "hero" for removing an extremely dangerous threat from the universes forever, even if it was simply because he didn't get invited to the party.