I don't get it

I'm really puzzled with all this ISIS stuff..

Over here in Britain, there are so many young people going to Syria to fight against US. They're being called 'British Jihadi's'

WHY WOULD THEY GO AGAINST US? We literally allowed them into our country, now they want to fight against us :@

I hate the scum and think they should just give up and kill themselves, they will all burn and die.