Why she's the killer: After Nolan cheats on Ava with another girl, it's possible that Ava wants revenge. Not to mention, she could be holding some resentment for having to keep their relationship a secret all this time.
Why she's not the killer: Even though she is hurt about their breakup, she looks visibly shaken up by his death. Deep down, it's clear that her and Nolan really did care about each other.