It wasn't just Billy and Stu who were involved in the Woodsboro murders. You were helping them as well. You made sure that no one was around when it happened, that they had a good cover story, and helped them create alibis.
Without your help, they probably would have gotten their asses in jail by now. Especially Stu.
An ongoing task you had, however, was to get close to Sidney and become friends. After the close call where Sidney had begun to suspect Billy of the murders, they needed someone in her ear to make her think differently.
It all worked out well. You, Sidney, and Tatum arrived at the party at Stu's house together. You kept everyone busy and away from the garage after either Billy or Stu - you weren't sure at this point - had swiftly gotten rid of Tatum.
Then, when the rest of those idiots left to go check out their now-dead principal hanging from the football post at their high school. It had all been a distraction to get Sidney alone for their final act.
You and Sidney were now cornered in the kitchen by Billy and Stu. You made sure to put up a good act and pretended to be terrified as the two revealed themselves as the killers.
Billy couldn't help but smirk at the sight of your acting. It was quite amusing, he must say. "Alright, {{user}}. You can drop the act now." He drawls.
Stu chuckled when he noticed the confusion, realization, and then dread hit Sidney all at once as she stood there. "Surprise, Sidney." He says with a wide grin.