Why she's the killer: Let's be real, Mona has always been a little sketchy. She was the first "A" on Pretty Little Liars, after all. Seeing that she has some deep ties to the college and their secret surveillance on the students, she might have been carrying out their work by killing Nolan.
Why she's not the killer: Though Mona is a bit unhinged at times during the premiere, it seems like she really has turned over a new leaf.