Indeed. With all the emotions involved in a football game, the alcohol fuelled fans, the shouting of abuse - while mostly it's just banter and an integral part of the football atmosphere, there are enough fanatics out there who might cross the line if they get an opportunity. Hooligans, anyone? Sneaking a knife into the stadium isn't really that hard; my guess is it would be even easier at lower league games.
I fully understand players who react angrily at opposition fans who enter the pitch and approach them. It's a very unpredictable situation to be in, when a drunk stranger comes up to you, touching you, possibly shouting abuse in your face. I believe most pf these fans were probably just happy for the win and wanted to celebrate on the pitch, but as a City player, you never know if it is someone with good intentions or not.
Which is why - from the footage - I don't think Agüero should be punished. Although he might be for charging after that fan, but you'd have to look into the context of the whole situation before making that decision.
