I do terrible, crazy things, and then you act all grim and gritty and stop me. And I go back to Arkham and you go back to... well, whatever it is you do when I'm not around. Brooding all by yourself in the dark, I suppose. That's just how we are. Or how we've grown to be. And it's never going to change.
~ Joker to Batman.
The Joker (real name presumed to be Jack Napier) is the secondary antagonist of the DC Animated Universe. He is the main antagonist of Batman: The Animated Series, The New Batman Adventures, one of the two main antagonists (alongside Lex Luthor) of the Justice League series, a cameo antagonist of the show Static Shock, the secondary antagonist of the film Batman: Mask of the Phantasm and the titular main antagonist of the film Batman Beyond: Return of the Joker. He is a completely insane and disturbed psychopath who seeks to cause chaos, destruction and misery wherever and whenever he can, as well as the archenemy of Batman.
Murdering Andrea Beaumont’s father even after his debt to the mob was already paid.
Attempted to bomb a child’s birthday party.
Murdered people with Joker venom.
Psychologically tormenting Charles "Charlie" Collins for two years as a "hobby" just because Charlie cursed him out, getting him involved in a scheme to blow up the Gotham Police Department, intending to kill Charlie as well.
Attempted to nuke Gotham City, despite Harley Quinn’s protests.
Used Ace, a young member of the Royal Flush Gang, in an attempt to have her use her telepathic abilities to drive 70 million people insane.
Brutally torturing a young Tim Drake to insanity and planting a microchip on him to take over his body.
While irredeemably evil in his later appearances, the Joker wasn't originally that bad in Batman: The Animated Series, where he was usually depicted more like a troublemaker than an outright murderer in the episodes he appeared in (although the bombs he used in the episodes "Be A Clown" and "Harlequinade" would have certainly killed people). It wasn't until Batman: Mask of the Phantasm that the Joker firstly showed his sadistic murderous tendencies in spite of retaining some of his comedic qualities. The Joker gradually became more dangerous and deadly in all his appearances from The New Batman Adventures onwards until he finally crossed the Moral Event Horizon in Batman Beyond: Return of the Joker.
The reason behind this is that Batman: The Animated Series was subject to censorship when it aired in FOX, as the network had strong rules regarding the show's content. As such, the Joker's plans usually didn't involve murder and his Joker venom could only be used as a mere distraction instead of a mortal weapon. However, since Batman: Mask of the Phantasm was a theatrical movie, the production team was able to skip these rules, and when all subsequent DCAU shows went to be aired on Kids WB and Cartoon Network, rules were less strict, allowing them to make more direct references to death and murder and make the Joker resemble his mainstream counterpart a bit more.