[Jamie]

Theory:

Spoiler!

One of the little girls is the one feeding rats to the walkers. She's also the one who mutilated the animal in the prison and murdered the sick people. Carol took the blame to protect her. It's probably the older one, but they could swerve and make it the younger one. There's a story line in the comics with a child psychopath who murders his brother and is killed by Carl, and I think this will be a take on that.