I agree. His intention was to kill a person who was bearing arms, despite the prohibition against infringing the right. It was I he thought he was killing, in the act of exercising my rights.
Had I been the one shot, many would agree that the shooting was justified since there is no way to know the intentions of a person bearing arms. You just get to assume that they mean harm and kill them.
In your case, I don’t think his intention was to kill a law abiding citizen. ;)