Cursitor Doom is cognitively challenged. The gunman was in room 32135 of th e hotel, on the 32nd floor. It was easy enough for the police to shoot him when they finally worked out where he was (which does seem to have taken a while - well-placed snipers are hard to locate). None of the concert-goers would have had much chance of locating the shooter, and few people carry th e kind of long gun (ideally with telescopic sights) that works for taking o ut a sniper.
The proposition that "a good guy with a gun" in the audience would have tak en him out is the kind of fatuous nonsense that Cursitor Doom find in the a lt-right media which he is gullible enough to take seriously.
He'd like the main stream media to feed him the kind of lies that he finds attractive, but happily he represents a rather small audience, and the main stream media prefers to cater to the rather more numerous rational majori ty.