I'm not one to pass judgement on what a player-controlled agent feels, or the the manner in which that character acts, so, I honestly could not care less if the kind-hearted nun has suddenly become a sadistic murderer because the player had a bad day.I just think there's a point beyond you have to start questioning whether the player is playing the character in a way that seems coherent and sane, and I don't think incoherent and insane PCs (or even, normally, NPCs) make for a good game for much of anyone outside of very specific contexts when everyone knows that's going to be a thing going in, and sometimes even then (Malkovians, I'm looking at you).
I find that enforcing any kind of prerequisite for reasonable roleplaying falls less under Game Master fiat, and more under "you're not playing your character right".
But, if your players enjoy it, it's not the wrong way to play.