Socks1000

Answer: I've known people who are influenced by a "leader" friend. They don't make many decisions for themselves. Stu seems to have latched onto Billy at some point, and is willing to join him in the murders. There are real-life cases in which murderers had associates who obeyed them.

Chosen answer: Billy told Stu they were going to be famous as the sole survivors of the massacre. He believed Billy and went along with it, wanting fame.