Biggest Piece of Evidence: Spencer seems very visibly affected when Mona admits to still having nightmares. Plus, Spencer herself admits to writing a criminology paper about a murder that fits these exact circumstances. The fact that she's so forward about it could mean a veiled sort of honesty. Why suspect the person who's giving you the most information?
The Point of Contention: Spencer is on a good path, getting her career in politics off the ground. And the truth always comes to light eventually.