You also assume that since he doesn't have a record that he has never committed a crime before. He doesn't have a record could just mean that he had never been caught. If someone I knew was stupid enough to try to rob a gun store, I would still believe that the person being robbed had the right to defend himself. Also, I can assure you that a person carrying any weapon in a business with the intention of robbing the place is there for a party. He is there to do harm. He took his own life into his own hands with what he did.

One more thing, a lot of people with mental issues still know the difference between right and wrong. That's why the courts still hold them accountable for their actions.