The second installment of Christopher Nolan's vision of Batman also brings us Nolan's vision of the Joker: a frightening "terrorist" with no political agenda other than to create chaos for his own amusement. Batman is certainly in this film, but it's the Joker who is really its center, its star. He laughs at his own actions, but in this film those actions are more terrifying than they've ever been. There's definitely something wrong with him, but what is that something?
The Joker's most likely diagnosis is antisocial personal disorder. To be diagnosed with that disorder, according to the Diagnostic and Statistical Manual of Mental Disorders (the diagnostic rulebook of mental health clinicians), an individual's behavior must meet at least three of seven criteria. The criteria most relevant to the Joker involve his repeatedly: 1. violating social norms of lawful behavior; 2. having no remorse for his misbehavior; 3. lying for personal pleasure or profit.
Nolan's incarnation of the Joker gets his kicks from wreaking havoc, and he's clever enough that there's plenty of havoc to go around; he knows what he can do and he's ambitious. He is grandiose, but does he have a second disorder — is he a narcissist? The diagnosis for narcissistic personality disorder rests on the notion that such people's beliefs about themselves are greater than are warranted by reality; they have an over-inflated sense of their own abilities. Unfortunately, Joker's beliefs in his talents are well-founded.
The Joker is frequently portrayed as the inversion of Batman: humorous, irrational, and spontaneous whereas Batman is humorless, logical and methodical. But what makes Nolan's Joker particularly interesting is his similarity to Batman: They're both smart, driven, methodical loners who are good judges of human nature. Unfortunately, the Joker, like some serial killers, uses his talents and abilities for his own sick amusement rather than for the common good, as does Batman. Thank goodness for Batman.