A lot of these people are just kids, and they don't think anything of smashing bottles into the faces of another person, including their school classmates. One of the most disappointing moments of my life, if not the most disappointing moment, was asking one of my classmates at school why he and his friends beat a bunch of drunk people, both male and female, with makeshift 'urban warfare' weaponry until they were bloody. He just grinned and replied "I don't know, it's just fun".
He later said "They're all moshers, who cares?"
That's right. They beat some people into unrecognisable pulp because they were wearing baggy clothes.
This behaviour of separation is nothing new, the 'alternative' kids have always been picked on in some way or another, but recently, in the last decade or two, the UK in-particular has been breeding this entire generation of kids with no brains or ambition by making celebrities of wastrels and glorifying binge drinking and drug use. As the education system crumbles into dust and the media continues to dumb down the population, these violent attacks have only increased and gotten far worse in terms of severity.
I honestly think it all stems from schools. The school yard is a violent place that rewards more violence. This isn't how children should be conditioned.
Edit: I try to hold a diplomatic tongue most of the time, but seriously. Jackie. Ultra Violence. Shut the fuck up, both of you. You're both intelligent enough to act the bigger man, so go ahead and try it.