Why She's Guilty: When you really boil everything down, Spencer is the smartest, most cunning member of the gang. She's dealt with a lot of personal issues, including temporary insanity. It's not a stretch that she'd be tormenting her friends after all this time. She also wrote that crazy Criminology paper with details that mirror the murder of Charlotte.
Why She's Innocent: Spencer's got a lot going on with her mom's campaign, and she's seemed pretty preoccupied by it. Also, now that she's hooking up with Caleb, could she really hide her evildoings from such a tech-savvy guy?