A couple of things I've seen recently have made me aware this is more common that I thought.
At the Sand Diego Wild Animal Park, they said that while male lions often live 30 years in captivity, they seldom live more than 10 years in the wild, because of injuries caused in fights with other males, among other things.
And a Nature episode about monkeys said a certain species in S America (Capuchin?) often battle each other in groups for the best feeding areas. More are killed by other monkeys of the same kind than by any predator.
Reason for pessimism about the possibility of educating the aggressiveness out of man.