Why he's the killer: Nolan is holding a pretty big secret over Dylan's head: that they secretly hooked up while Dylan was in a relationship with Andrew — right before they were going to move in together, mind you! Ever since, Nolan has been blackmailing Dylan by making him do all of his school assignments. Maybe Dylan finally hits a breaking point and snaps!
Why he's not the killer: It definitely seems like Dylan has some unresolved feelings for Nolan (he might even still be in love with him?), so we doubt that he would actually kill him. Not to mention, he is the first person to jokingly suggest to Ava and Caitlin that they should kill him. That would kind of give him away, no?