Question: Blaire was responsible for filming and uploading the embarrassing video of Laura, which led to Laura's suicide. Aside from Blaire, how were Mitch, Adam, Jess, Ken, and Val involved with the humiliating video that made Laura kill them?
Answer: Blair's friends were responsible for some of the online bullying, but she said everyone is doing it, so they did. That's the only thing I can think of anyway.
Answer: Given the scene at the end of the film (the computer slams shut and Laura attacks Blaire) is in a POV, it should be assumed that it takes place through the eyes of Blaire.
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