I agree and as above, anything said regarding these chants isn't aimed at Utd or Utd fans as a whole but the minority of idiots that sing them.
I think you've touched on it before, the second the whistle blows, something changes in the mind of a section of fans and they believe it's ok to sing and chant certain things. I've seen it first hand at Liverpool - Everton matches. Before the match, at half tiem and after the game you've got friends (possibly family members) shouting back and forth to each other, arranging what they're going to do later that night, but during the match these very same people are slinging insults at each other.