Would you kill a man who's robbing you? (long)

[BCPVP]

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In the scenario described, up close like that, a knife is really gonna ruin your day.

With the gun culture in most of the Western world, people seriously underestimate edged weapons and the damage that they can do.

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This is pretty true. At the range described in the OP, IF the robber was willing to kill the OP if he resisted (debateable considering he knew the OP), even a gun in the room might as well be in the store it was bought from. I believe the police can shoot someone with a knife that is within 20 feet of them because they constitute a deadly threat.

So the only other option is to shoot him as he leaves which, I think is wrong because he is no longer a threat.