An armed man comes up behind a customer at the pay window of a Detroit gas station and orders him to empty his pockets. The gas station employee was safely locked behind bulletproof glass. The police were nowhere around. Even if the gas station attendant called 911, they couldn’t possibly get there in time. And the victim could not know if he was merely going to be robbed or much worse. He was at the mercy of a gunman pointing a weapon at him.
He was told to take everything out of his pockets. He removed his handgun first and killed the mugger with it. Problem solved.
Jesus said that he who lives by the sword dies by the sword. Obviously, the mugger was trying to make a living using his “sword” to expropriate wealth from others (just like the government). The armed citizen, not being a criminal, did not “live by the sword.” He was just defending himself.
He also had an obligation to not allow the robber to take his gun and use it to victimize someone else. If he had complied with the criminal he would have put another gun on the street.