There's an assumed story to what really happened to Heath Ledger. The abridged version of the tragic story states, with some confidence, that Ledger lost himself to the intense process of becoming the Joker for Christopher Nolan's The Dark Knight. He allowed the Joker into his mind too much and what stayed there even after he was done with the role broke him.
The reality is more nuanced, more complex. But then, talking in such absolutes feeds into the mythology of the movie. It's vulgar, but having that sort of emotional production story is the kind of thing Hollywood falls over itself for. It's not right, but the casual, indirect exploitation of tragedies like this and Paul Walker's death feed into a film's selling power. And when the news broke that Ledger's approach to the Joker might have had something to do with his death, it gave the performance a gilted edge before anyone had even seen it.
The controversy behind the performance became a currency to such an extent that it didn't matter how little Warner Bros or the Nolans played into it. The potential audience did all the work for them.
Ultimately, this is the story of how a young, talented actor on the verge of breaking into the very upper echelons of Hollywood died of an overdose in January 2008, a little under six months before The Dark Knight came out. It is one of profound tragedy, huge artistic skill and the commitment to a role that was so remarkable that it becomes difficult to discern whether it was responsible or whether it ought to be celebrated away from the context of Ledger's death.
Given the focus on how the Joker impacted Ledger's life and death, it's important to tie this story to the genesis of the character.