There are few boxers who I admire. But my favorite was a feather class professional boxer from my city, Muhammad "Alfa" Alfaridzi. Different than other pro boxers that look 'hard', Alfa was a cute guy, came from religious Muslim family, with humble personality and modest appearance.
His achievement in pro boxing was good enough, and many people expected him to become world class boxer. But Allah determined differently. Alfa died on April 2, 2001, three days after he knocked out in his last match against Thai boxer. He was 25 when he left this dunya.
Brain injury. That's what happened to Alfa. The factor that makes professional boxing can be deadly is because the boxer head is the main target of punch, while professional boxer don't wear head protector like in amateur boxing.
So, I can understand if there are ulama who against professional boxing.