That said, I think everyone is entitled to express their emotion and opinions. Of course you're not going to open a medical textbook to use illnesses to insult him, but booing is the most non-rude way to voice that you dislike someone in a crowd.
I do not have time to carefully explain why I do not like player X or Y. That'd take too long. I just shout boo when they come on if I feel like it. Can't take it? Get back under your rock as it happens in each sport and to deny people to be passionate while trying to make them passionate is hypocrit as fuck.