Honestly, I'm just glad we have gun laws, I'd hate to see what could happen if hot heads like that were allowed to walk around armed!
I'm actually quite surprised this has never happened to me. I'm too lazy to get out and abuse someone one but I'm not beyond flipping the bird, mouthing 'fuck off' to someone and even being a little annoying. My wife has told me heaps of time not to antagonise the angry fools but sometimes it's just too funny not too.
At least they have the ability to put new (old) songs in their set because they aren't stuck playing 30 minute sets where the crowd is there to see someone else.