Is there even a mental illness that could make someone do the things the Joker does?
The phrase sociopath gets kicked around alot, the symptoms of which are...
1.Failure to conform to social norms with respect to lawful behaviors as indicated by repeatedly performing acts that are grounds for arrest;
2.Deceitfulness, as indicated by repeatedly lying, use of aliases, or conning others for personal profit or pleasure;
3.Impulsivity or failure to plan ahead;
4.Irritability and aggressiveness, as indicated by repeated physical fights or assaults;
5.Reckless disregard for safety of self or others;
6.Consistent irresponsibility, as indicated by repeated failure to sustain consistent work behavior or honor financial obligations;
7.Lack of remorse, as indicated by being indifferent to or rationalizing having hurt, mistreated, or stolen from another.
Which at first glance seems to describe The Joker pretty well, except as the movie shows the Joker is a master at planning things out and is patient enough to let his plans play out and wait for the right moment to do things.