Why It's Him: Mike hasn't been in Rosewood for a while, but he has a very troubled past. Let's not forget that he used to break into neighbors' houses and has always had issues with his temper. He's been romantically involved with Mona, which would make it easier for him to turn against the liars. And besides, maybe he hasn't been so absent after all. Maybe he's just been hiding out, waiting for the moment to strike.
Why It's Not: Mike always seemed pretty harmless, you know? Like, yeah, he had some trouble here and there, but he's never seemed capable of harboring a serious vendetta against the liars. Plus, the fact that he hasn't been around could just mean he's gone for good.