Prewitt used to be an awesome boxer, but when he blinded his friend during a casual sparring match, he decided it was time to quit. When Captain Holmes wants Prewitt to fight, Prewitt explains what happened, but Holmes reacts with callous disregard. He says, "You might as well say stop war because one man got killed." Holmes decides he's going to force Prewitt to fight, bullying him into it.
Prewitt tells his girlfriend Alma about what happened after he knocked his friend out:
"He was in a coma for a week. Finally, he did pull out of it. Only, the thing was, he was blind. I went to see him at the hospital a couple of times. Finally, I just couldn't go back. The last time he and I started talking about fighting and... He started to cry. Seeing tears coming out of those eyes that couldn't see anything... I thought I ought to tell you."
The whole incident serves to illuminate Prewitt's attitude, and the way it differs from that of a jerk like Holmes. Prewitt, while tough, still has a lot of natural human sensitivity—Holmes isn't particularly tough or virtuous in any way, and he's cruel.
So Prewitt, while a warrior, doesn't love violence and cruelty—he wants to minimize its role in his life, chill out, and play the bugle. But, like Al Pacino said in The Godfather Part III, "Just when I thought I was out… They pulled me back in."