I think you have to hate Liverpool with a passion to be considered a memeber of the human race but I'm not sure if it is compulsory for a United fan (I forget what is the difference between a fan and a supporter again?)
It's all very confusing, but I think that if your a "follower" of Man Utd, or "Neutral Fan" to use the correct terminology, you don't have to hate anybody except Ruud Van Nistelrooy and Gutsy