Soccer or Footbal

This has nothing to do with either of the sports, in fact it has to do only with the fans. It's something I've been wondering over lately, specially since a youn man was killed by the fan of one soccer team; the crime: wearing the T-shirt of a soccer team different than the one the murderer likes.

It's such a senseless crime, a waste of life on the account of something designed to entertain and bring people together; it's the crumbling of sports into a reason for murder... Anyway, what I've been pondering is the differences between the fans of soccer and those of football; maybe because I don't know much, but every so often I read on the news about riots caused by Hooligans, and other fan groups of soccer teams, but I still haven't heard that from the fans of Football, even if it is a sport apparently more violent than soccer; why is it that a mild sport (in terms of physical violence) has such violent fans, and a much rougher one has more calm fans? If anyone has an idea on why this is so, please let me know, for I can't think of an explanation for it.