The key part of the Joker's success at the beginning of the film is the misdirection of the scene. The film opens on a bank robbery, perpetrated by nameless men in masks. They all seem like they're following the orders of the Joker, who is running the crime unit. But then the tables are turned, because the Joker turns out to actually be on the scene, pretending to be one of the anonymous lackeys. And the body count quickly escalates from there. Is there a very good reason for him to have pulled that stunt? No, not really. Unless you consider "just feeling like it" to be a particularly compelling excuse. And that's the Joker in a nutshell. He's an anarchic, chaotic force of nature, as unpredictable and senselessly violent as a bolt of lightning. It's hard to figure out his motives when he doesn't really have any -- as Alfred poetically reminds Batman, "Some men just want to watch the world burn."