That's terrible stuff.

They all seem very drunk, and that includes the victim. It's a really bad idea to be very drunk, alone and male. (and with your radar totally shut down, as Neal pointed out) That combination means a very real risk to get into all kinds of trouble at night and a high risk of being a victim to robbery and violence.
It's not easy to defuse a gang like that, that would need coordinated action of some kind. If you do anything alone they will have a go at you all at the same time. You can see a few persons protesting and trying to hold things back, but you can't talk to guys like these cause they punch anyone who oppose them. So that means it's all or nothing, a few of equal size and very quick and determined guys, preferably not drunk as people are terrible at fighting when drunk, could contain these idiots. But that won't happen very often unfortunately.
I remember once riding the tram when a drunk man started to verbally abuse me. I refused to give in to his insults and threats and I didn't move. He was clearly threatening, although I wasn't afraid as I was totally sober and this man had problems even walking, so a fight would have been a joke. The point is, no one did a thing for 5-10 minutes. No signs of support at all. The person next to me just upped the volume of the mp3 player... People simply ignored what was going on, because if they said something they feared to be the next target of this guy... Eventually this very old lady with her handbag in her lap had had enough and told this guy to shut up!

She was the only one that did or said anything! All my respect to her. At the end of the day I actually felt more like telling the other fellow commuters how incredible spineless they were, rather than jump on the abuser...
A gun in a bus with drunk and scared people in the way? I don't know if it's a good idea really... I can't imagine it is that easy to control something like that with a gun, and if you act on your own, who knows if someone else pulls out a gun. But it's certainly better than people end up getting kicked to death or get severely handicapped for the rest of their lives by a bunch of scum bags.
cheers porphy