Like in the case of UCLA professor, let's see how carrying a gun could have saved those in the club.

Let's assume everybody has a weapon and there is no limit on what kind of weapon it is, because, according to those weapon advocates, if there is any rule, that is going to ONLY limit the good guys, because the bad guys won't follow, so let's stay with no limits, to be fair.

1. The club won't be weapon free, everyone carries a weapon, to be convenient, a gun (feel free to carry an assault rifle, which will be proven effective later).
So most of the days peace prevails, though it is quite awkward dancing with a gun, it is safe... until there is a chance for fist fight, it all turns into gun fight. After all, who is going to give up the chance to win, especially under the influence of alcohol.

2. Now the terrorist came, with heavy assault weapons, because he knew this is not a weapon free place, as a matter of fact there is no weapon free place, because, according to weapon advocates, those places are welcoming bad guys, hence totally gone after they learnt the lesson the bloody way.

No, the bad guy knew there are guards with weapons, but having weapon and be trained to safe guard people makes no sense to the bad guy(otherwise the notion that having a gun can stop violence will make no sense), and the good guy for that matter, so everyone needs a weapon and has a weapon.

So the bad guy is well equipped for the worse situation.

Now the battle starts.

If you have a brain and it actually works, you will know how much will be killed, I am sure that the bad guy is long gone before the police came, but imagine how long it takes him to shoot the crowed before he himself being killed, 50+ be killed won't be hard, he could have explosives, grenade or even a nuke if he likes. What? Don't worry, weapon ban only works on good guys, the bad guys can get anything they want, as long as they want it.

Let's think now how effective on life saving in this case when everyone in the whole country is having a weapon, remember, it could be 2 or 3, there is no limit.