I think I'd prefer the safety valve working the other way.

I agree that a safety valve that makes sure only Good Guys (who?) kill Bad Guys (who?) would be more morally valuable. But if you're doing that you might as well program moral laws into the universe itself, so it is impossible to lie, steal, or murder.

This is a technically feasible (but this is magic we're talking about) hack which makes it more difficult to mistakenly murder or torture people with magic.