Biggest Piece of Evidence: We know Sara is still around, as evidenced by her dramatic appearance at Charlotte's funeral. Of all the characters to return so far, she seems, by far, to be the most malevolent.
The Point of Contention: According to the police, Sara's burns — which we still haven't gotten the full story on — eliminate her as a suspect. Charlotte was violently attacked before she was thrown from the bell tower. Sara simply didn't have the strength.