Two words, Straw. Man.
That scenario is so out of whack it isn't funny.
A straw man argument is an imagined scenario, so ridiculous that it would be highly improbable to impossible in real life, created to support one side of an argument.
Key phrase "so ridiculous that it would be highly improbable to impossible in real life"
The argument of your idiot acquaintance is rooted in fallacy, if 200 gangsters were outside of your house then they are there for a reason and regardless someone would phone the police who would consider a mob of 200 to be a danger to the public and respond in kind, expect something on the news about 200 gang members killed in a shootout with the police/National Guard.
All that being said, I'm taking as many of them with me as possible, it really doesn't matter what anyone thinks at this point, only that the number of evil dead outweighs the number of the righteous, and if we can kill em all, then oohrah.
About your acquaintance, rational, logical thinking doesn't work for most of these people, personally I think they are mentally sick and should be committed to an insane asylum. I mean really, what other reason aside from insanity could someone have for thinking like that?