The robber brought his own death upon himself. If he hadn't been in the process of committing a robbery, he wouldn't have placed himself in a position to be killed. The Bible says, "If you live by the sword (or gun), you die by the sword (or gun)".
When a robber walks into a store with a gun with the intent to rob it, and potentially harm those inside, he is taking his own life into his own hands. He doesn't know if the clerk (or one of the customers) has a weapon, and he doesn't know if he will walk back out or be shot. All robbers are aware of the danger. The clerk and the customers are under a threat of violence by the presence of a weapon. They have a right to defend themselves, with deadly force, if necessary.
The guy robbed the store before. Just because no one was hurt that time doesn't mean someone wouldn't be hurt the next time. In fact, the odds are greater that someone would be hurt. I'm not rejoicing because this young man was killed, but his own actions are what caused his death.
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