Why she's the killer: We can't help but find it a little weird that Alison leaves her life with Emily behind for a position as a TA, even if their relationship is complicated at the moment. Maybe her move to Beacon Heights University is about more than just being a mentor to young students. She is extra pushy about getting Caitlin, Dylan, and Ava to distance themselves from Nolan. Perhaps when they don't take her advice, she takes matters into her own hands.
Why she's not the killer: That being said, it's pretty unlikely that Alison is the killer. Not only does she seem to genuinely want to help her students, but she's one of the OG pretty little liars. If she is truly a killer, wouldn't she have done it by now?