When asked to think of an evil comic book character, there's a 99.9% chance that the Joker is the first character to pop into your head. With so many of DC's most depraved comic moments being the work of the Clown Prince of Crime, it's inevitable that he'd become synonymous with doing bad things, as when you see him in a comic there's a strong chance that's exactly what he's up to.
But having Joker only do evil deeds would really make for a dull character. One of the most exciting things about the Clown Prince is that he's unpredictable, and that you're as likely to get a weird knock-knock joke and a pat on the back from him as you are for him to remove your frontal cortex.
True to form, this means that sometimes the clown does genuinely heroic things - such as the fact that he's actually been responsible for saving the world at least three times now.
They're not always done for the right reasons, but even with this in mind there's still a shocking number of occasions in which Joker was arguably the hero of the story - whether we like to admit it or not.