What killed him in the end

My simple explanation , of what killed him in the end, simply he became the characters, he really inhabited the character, and lived the role.

Just take a look at the production of Brokeback Mountain and what that did to him, psychologically , it wasn't just another film role where he reads the lines off the page. That movie literally wrecked him emotionally and psychologically and all he did was act in it. Because he was in the MIND of the tormented character the entire time

And you can take almost any of his films and find the same trajectory.

Most actors, are not capable of that. Most actors don't even TRY to get to that level.