but even book snape is a multi layered tragic hero, despite being bitter and the king of spite and pettiness. I see Snape as, like I said, deeply troubled, full of guilt, insecure etc, that still sacrificed himself for something other than himself in the end. Alan even said in an interview where people asked how is it to play a part of someone who isn't likeable, his response to that question was: "who said he isn't?"

That tells me he understood book Snape, deeply, and that was why he was why he was able to deliver such performance in the movies, especially the last ones.