They should learn to appreciate natural selection.
Someone threatening another with a gun, replica or whatever, then not complying or threatening officers... Well, he's dumb and while I don't wish that person's death, I won't mourn him either. He just had to use his mind. If he had one.
Actually, a replica or low-power airgun simply makes this situation dangerous for the criminal himself and no one else, as assuming he decided to shoot, he wouldn't be able to cause serious harm to anyone.
The only situation I can see that could cause a problem IMO is if that person was a kid who doesn't really know what he's doing, and in that case in comes down to responsibility from people around (friends, parents, family,...) to put any airgun/replica out of the kid's reach, and properly store it.