Apparently, crazy is as crazy does, and needs no further explanation. The Joker is first introduced in TDK rather brilliantly, as dangerous, psychotic, and reasonably well-played by Heath Ledger. The problem is, other than his own dialogue in 2 moments of the movie about his father that he hated who gave him is facial scars, and some rambling dialogue about some female he was in love with who seemingly experiences the same fate (it was a bit jumbled), there is no other background about him as a character. He just exists, and has no rhyme or reason, no motivation, other than he wants to kill “The Batman” — but then later he… uh… doesn’t want to kill “The Batman”.
Perhaps this equates to how the writers perceive people being disfigured as immediately and definitely correlating with them turning into homicidal maniacs. Harvey Dent was the epitomy of do-gooder, until 1/2 his face burned off. Insta-crazy!