3) He Once Had Reality-Warping Powers
Have you ever wondered what someone as crazy as the Joker would do with actual superpowers – say, the cosmic power of the Green Lantern or Superman’s abilities? Well, in the “Emperor Joker” storyline he became more powerful than them even.
Stealing the reality-warping powers of inter-dimensional Superman villain Mister Myxyzptlk, Joker remakes the universe in his own image – killing Batman daily only to keep bringing him back to life. He even plans to destroy the whole of existence. His undoing, however, is his reluctance to permanently eliminate Batman. During a moment of hesitation, Superman is able to defeat him and his powers are removed.
Nevertheless, the harrowing experience leaves Batman near-insane, until Superman finds a way for Bruce’s memories of the event to be transferred to him and Bruce is cured.