How many Jokers exist out there? Frustratingly, and possibly deliberately there is no psychological assessment of this maniac - the more or less good guys, in this shades of grey movie are too busy fighting him to figure him out. He has no history and he seems to make stuff up. Abusive father? Lost love? Irrelevant?
Thinking of burning the world on a large-ish scale, the obvious examples are the LTTE and maybe Al-Qaeda but both those have reasons. What they've done is awful, but waking up at 3am, there are rationalisations which soothe their conscience, even though it may not seem justified to others. Surely everyone has a price in a broader sense of the word: something they want, not necessarily money. So then the Joker wanted chaos? A world tearing itself violently apart satisfied some need for ugliness? For everything around him to be as scarred and freakish as he is? He's just 'a dog chasing a car', no rules, no plans, no 'end' as such just any means necessary.
Can't think of anyone like him, yet. Can you?