Why She's Guilty: They often say the person closest to the victim is the most likely suspect, and no one was closer to Charlotte than Alison. We know Ali is a masterful liar and manipulator. Maybe she's playing one last game with the gang. Maybe she's faking all this grief and sadness to deflect suspicion.
Why She's Innocent: Why would Ali push so hard to get Charlotte released, only to turn around and kill her? Ali truly seems close to Charlotte, and it seems strange that she'd stick around Rosewood for five years, just to pull off a murder.