I think in sports, obviously its fine to root against other teams. Thats what sports is, wanting your team to win the other team to lose, Your team to play well the other team to play poorly.
That said, you start to cross the line when you want a guy to suffer a gruesome injury or feel joy he fractures his ankle in front of millions of people. There is a saying I like that says the moment we stop caring about one another is the moment we lose our humanity. I consider my self a HUGE Jazz fan. I love the Jazz. Having them in my life since '97 has been awesome. But I don't think I have to sacrifice my humanity to maintain my Jazz Fandom.
Sports is suppose to be fun. There should be an element of good naturedness to it because in the end we are all still human beings and there are things much bigger than sports. Think about the mentality you are promulgating for a second. It's a blatant disregard for ones well being. It's villainous and barren of decency. And the world is saturated in such things enough. So much suffering in the world already because of it. Nothing should get in the way of good will toward others.
Yeah, let's cheer against opposing teams and their players, but in a good natured way. Good vibes toward everyone. And I think feeling joy for someone getting hurt is just the gate way mentality to fostering a desire to hurt others. Compassion, kindness, empathy, good will should win the day. It's just a game. Nobody needs to hate someone else or wish them harm because of it. That's a bad place to be.