Why She's Guilty: Aria is the only person to vote against Charlotte's release. She even says she'd be afraid for her safety if Charlotte was released. Furthermore Aria, like Ezra, is actually not totally accounted for the night of Charlotte's murder. We can place her here and there based on her testimony, hotel security tapes, etc. But do we really know for sure? Once upon a time, we really thought this one was "A."
Why She's Innocent: Aria seems just as terrified of Ezra as the rest of the crew. Plus, hotel footage does place her back in the hotel before Charlotte's death.