In starring as The Joker, Heath Ledger knew everything.

That's not to say that "learning" caused his demise, but rather that it 'made him ready' for it; as Jesus's hallucination on the cross made him ready for 'it' to be 'finished.'

Ledger's performance in The Dark Knight seemed to portray a fuller understanding of 'the human condition' than most comic-book movies display at first glance.

It was as simple as the 'I'm the bad-guy'-declaration a few movies' villains have had in the past, but more; it was also a declaration that 'I know that stuff turns out with the good-guys winning; so if I'm absolutely evil, then the good turns out even better! Why not have fun being evil while you can?'

You know and I know that 'good'–while definitely not as excitingly unpredictable as 'evil'–is what survives the shaken-together filter. Life is not worth living if your works will not survive. For Heath, the thrill of the drugs was worth more than the chance for any future works; but his Joker will live on.