The Motive

Billy wanted to kill for two reasons. First he killed Maureen Prescott because she was having an affair with his dad which caused his mom Debbie to move away. He tried to kill Sidney but failed. The second reason is that he wanted to create a sequel because being brain-washed by scary movies.

Stu obviously had a different motive. He mentioned it was peer-pressure, but he was probably not that serious about it. Stuart was also messed up by watching too many horror movies because he did not mind killing close friends. His reason was since his head was so far out of wack that he really wanted to make a real life scary movie based on the horror movie rules.