Roberts did steal the sheep, though. That's the problem. He took Bertia's prized sheep and killed it, and then stuffed it into the coffin to hide the fact that the corpse was stolen by someone else (which he received money to keep quiet about).
While he did try to make good for it by confessing and returning the money in the end (and only if you manage to pry it out of him), he still did all those things, and once you tell Bertia his fate is in her hands.
