Why cant football fans behave like normal people? What sort of mentality goes to a football game with that sort of mentality?

I see Sottish fans complaining that it's unfair because they cant sing their sectarian songs, even saying it has nothing to to with the government or police. Incredible. You cant walk down sauchiehall street shouting sectarian insults and saying you want to kill people because of their religion or beliefs so why should it be ok to do it just because youre in a football stadium.