These theories are just Scream fan-theories of Who Killed Who.

Theory #1

Audrey Jensen and Piper Shaw's initial relationship was that Audrey "let Piper in" on how to access all the towns dark and dirty secrets. Audrey being an outcast found someone to confide in and shared the Malware that let Piper delve deeper into the town. Piper was looking for someone to help her learn more about the town where her Dad came from. Eventually Piper learned who Emma Duval was in relation to her.

Audrey's viral video was posted. Audrey was pissed and Piper uses Audrey's anger to coerce her into scaring Brett Shimura and Tyler O'Neill. Audrey does the initial phone and video scare while Piper kills Tyler (unknown to Audrey at the time). Piper eventually takes over the phone work using Tyler's phone which she now has in her possession.

Audrey doesn't know that Piper took it to the next level with actual murder initially. Audrey gets spooked, Piper blackmails Audrey to continuing their work together namely continuing the video stuff.

Piper kills Rachel Murray to prove to Audrey she's serious and also to resolve the lose end of Rachel. Just in case Rachel suspected something based on Audrey's mood the night Brett and Tyler were murdered. Killing Rachel also takes away any alibi Audrey had further dragging Audrey into Piper's plot.