Hatred For Predators.

It is clear that he is not fan favorite material. But his competitors still didn't stop frowning at him. It is mainly because, just like the spectors, they hated him to their core. 

Unlike the spectators, they had done their research on him. They knew him for the predator that he was long before he was introduced. Many of them have issued challenges to him in real life. Many of them also want to kill him for the atrocities that he has committed. 

Be that as it may, they are also afraid of him. It is because he should be in the top three in terms of total destiny alone. He is an undisputed threat to all of them. And by the records of what they know about him, they can be certain that he will come after them. 

So they don't like him at all. It is understandable. The threat he posses to them is not something that the booing crowd can change. He will still gain more fans and become stronger if he performs as he usually does.