Andy, I think the basic mindset of hurting someone in the name of sport is simply down to competition, to be the best. I don't believe it's the ingrained desire to hurt someone that is the pull factor in boxing, it's the sheer machismo of being the strongest, the toughest and, certainly in lower weights, the best boxer.
It's the same in any sport, I play rugby, in everyday life I'm a nice chap who wouldn't say boo to a goose, but when I'm on the pitch, the 6'5" hulk running right at me has to be coming out of our challenge in pain because if he doesn't a) he'll keep running at me b) he'll have the mental edge when I run at him, at the end of the game I'll hope he's not hurt badly but the simple matter is, he's challenged me and one of us have to win, most of the time it's the 6'5" hulk but I'll at least have a go! That's what sport is about!