You know, I know that misunderstandings are the point of series like this, but I can't help but feel bad for Will. Until he finds out the truth about slime-jelly (which I wouldn't be surprised if he never does), he's gonna be under the impression that he almost killed his sister by almost killing the one person who could cure her, and that he's a monster for trying to kill someone who was "actually willing to turn over a new leaf". But in reality, he was completely justified, and going through with the murder was the only thing that actually allowed him to save his sister, even if it wasn't because the hooded man helped him out.