Most participants in close combat are literally frightened out of their
wits. Once the arrows or bullets start flying, combatants stop thinking
with the forebrain (that part of the brain which makes us human) and
thought processes localize in the midbrain, or mammalian brain, which is
the primitive part of the brain that is generally indistinguishable from
that of an animal. It may be that in conflict situations this primitive,
midbrain processing can be observed in a consistent trend toward resisting
and avoiding the killing of one's own species. During territorial and
mating battles, animals with antlers and horns slam together in a
relatively harmless head-to-head fashion, rattlesnakes wrestle each other,
and piranha fight their own kind with flicks of their tails, but against
any other species these creatures unleash their horns, fangs and teeth
without restraint. This is an essential survival mechanism, which prevents
a species from destroying itself during territorial and mating rituals.
Lt. Col. Dave Grossman, On Combat
http://tipyomi-sender.appspot.com/showtip/Sheepdog%20Tip%20of%20the%20Day/During%20Combat/74