Question: Why does Stu go along with Billy's plan? He's got nothing to gain. That and why kill Sidney? She can't help what her mother did with Billy's father.
Rob245
11th Nov 2019
Scream (1996)
Answer: Some people are influenced by a "leader" friend. They don't make many decisions for themselves. Stu apparently 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.
Answer: To add to the other answer, Stu briefly mentions "peer pressure" as his reason for going along with Billy's plan. Sure, he's being slightly sarcastic when he says it... but I think that it's a hint to his motivation. He's already somewhat unstable (his hyperactive personality throughout the film backs this up), and I think the implication is that Billy gave him the final "push" that put him over the edge into full-on violent insanity.
Answer: Stu is simply crazy. Probably brought down by Billy, sucked into his psychosis, sometimes that can happen if Stu is mentally unstable and easily manipulated. We don't know where his instability comes from, but its positive Billy has had a bad influence on him and brought him down this path of killing.
lionhead