In the flashbacks near the end showing that he did act as lookout changed the entire story. If he did act as lookout, he was obviously in fear of what they would do to him if he didn't. If so, he was not responsible for the robbing or the killing, that would have happened with or without him. He was a good kid, who was bullied into doing a bad thing. These things happen all the time. It is indeed a slippery slope.
Review of Monster
