I went in with my biggest fear being that it would make the Joker sympathetic and less of a villain. He needs to absolutely be a hate-able supervillain. I don't think the movie made a "hero" out of him at all, except for maybe a "hero" to other villains.

People are affected by their context, for sure, and the movie paints a context for how someone could become a Joker. But it certainly doesn't make him not responsible for his actions. The stunned feeling the audience has when he shoots Murray is evidence of that. Most people aren't cheering for him, saying, "Yeah, Murray DESERVES to die!" If the movie had portrayed a character as a heroic protagonist who is not responsible for his anger, then we would have felt a lot more catharsis at Murray's death. Instead most people watching the movie are stunned and uneasy, because he is escalating and doing terrible things.

He is portrayed as a villain. It just gives us a story for how one might become such.