Because it's largely impossible to find out whether people believe nonsense or just pretend to, one should where possible avoid needing to know. If you can set things up so that the consequences for Jim killed the puppy plus either "Jim believes God told him to kill the puppy" or "Jim pretended God told him to kill the puppy" are roughly identical you needn't waste your time trying to see inside Jim's mind.

This is trickier with retributional justice. If you think your justice system's goal is to punish wrong doing it's difficult to argue that it doesn't matter what Jim really believed, as if he really believed God was telling him to kill the puppy how can that be wrong?