I think that in most jurisdictions, it wouldn’t be legal to take someone’s gun and then shoot them with it. Once they’d been disarmed, they may no longer pose a threat. However, if you pulled out your own gun and shot them, this would be self defense, as they were still a threat when you attacked.

Morally, I think many religions claim they’re objective and say that killing is wrong. However, they often carve out exceptions.

My own subjective morality in this case would agree that this is immoral, as the robber has been disarmed. However, its impossible to be sure they’re really no longer a threat.

However however however.