It would go really, really bad. Not everyone is willing and able to effectively use a gun, and if everyone had a gun in that nightclub, it could have turned out worse just as easily as better. You're in a confined space with low lighting crammed full of people who have been drinking and all have loaded weapons. That seems like a disaster to me. The shooter opens fire, a couple people see it and shoot at him and then others don't know who the original shooter is or if there's still a threat.
You may not be able to get rid of all guns, but you're making it insanely hard for people to get guns. If it's harder to get a gun, then they may reconsider their method or even not carry out the plan in the first place.