Following the events in Paris, I have read multiple right wing commentators who speculated that if only Parisians had had guns, there would have been a very different outcome at the Bataclan. Well, they're probably correct, but I don't know that the different outcome would have been a better one.
I won't argue whether or not a gun ever helps to deter crimes. I understand there is good data that points that way but there is also good data that indicates that the very presence of a gun makes things more dangerous. That is what I expect would have happened at the Bataclan.
Recently in less crazy settings we have seen incidents that went horribly wrong. There is the story of the car jacking victim in Houston a few weeks ago who was shot in the head, presumably by a "good guy with a gun" who was trying to help stave off the crime. Instead, the victim was shot, the carjackers escaped, and the "good guy" picked up his shell casings and fled the scene, no doubt worried that he would be arrested.
I'm reminded of the meme that went around a few weeks ago... A small boy was throwing stones at other children in the school yard. The teachers knew that something needed to be done or one of the children would likely be injured so, they handed out more stones to the other children. That makes sense...
Adding more guns to a terrible situation does change things... but not necessarily in a positive way.