9. From Killer To Goofy Trickster To Psychopath
One of the most frightening things about the Joker is his unpredictability. Two ideas have been put forth to explain how Joker changes personalities so dramatically. One is super sanity. The Joker realized that the whole world is one big thrill ride and his insane choices are his way of jumping off the ride. The other is that there are three separate Jokers: a killer, a clown, and a psychopath.
In his initial appearance, Joker was a serial killer and jewel thief, distinguished by his garish appearance, his sad grin, and his use of a version of Joker Venom as one of his methods of murder. Joker seemingly died after only a few stories. When he returned, it was as a goofy prankster that engaged Batman and Robin in often silly, prank-filled capers that involved giant props and prop weapons that were often harmless. This Joker also dabbled in super-heroics, occasionally aiding superheroes against his fellow villains.
The next time Joker emerged, he was a vicious psychopath. Killing indiscriminately, often just to make a joke, he fully embraced the insanity. Joker made it his mission to drag Batman down to his level. He has done heinous things to the members of the Bat family, including crippling Barbara Gordon and killing Jason Todd.