Re: Hormonal & Psychological Responses to Combat
I agree that cultures have evolved, but I maintain that the human gene pool (and the behaviors it engenders) has not done so appreciably, and that we still have the same potential for killing hardwired into us.
As you note, however, we have cultural controls to alter the way in which that potential is expressed. Human behavior is fluid because our genes permit it to be so. Where it is not beneficial to kill, complex social constructs can keep it under control. In a society where it still may confer a survival-reproductive benefit to act directly on aggression (e.g. village raiding and warfare), humans still kill as a natural event.
I believe that social control system vulnerable to breakdown under the stresses I mentioned previously. Note the looting, raping and killing that occurred in the aftermath of Hurricane Katrina, and that which is currently happening in Haiti after two severe earthquakes. Once societal structure and order are shaken, the fabric of society is torn and survival mode draws on primal instinct among the most desperate and the opportunistic.
By the way, a number of mammal species' males kill the offspring (and even fetuses) of rival males. Horses are known for this, with a stallion inducing abortion in mares pregnant by a rival, by kicking their abdomens after driving off the rival and taking over his harem.
Bonobos may be non-violent, but that coupled with a lack of weapons technology, in the evolutionary picture may spell their doom in the face of armed, aggressive humans in search of bush meat. :-/