My current theory is Netina. She fits the description, was "close" with Mary at some point, and has a motivation for getting Reginald in trouble. She also seemed a little too eager to kill the frog in chapter 1, even if she said it was a joke.
We've known she was a sociopath since chapter 1. Seems a bit too obvious. Plus she's the most unlikable so there would be far less emotional impact than any of the other characters.
