[These thoughts] mesh with what I have written (making my comments a little redundant!
Now I must disagree; I thought your comments added quite a bit.
Aside from the question of physical evolution, however, I tend to see cultural and social conditioning as akin to ecological pressures in encouraging or discouraging aggression between members of the same species.
I agree that physically we probably haven't evolved much in the relatively short time since the neolithic; but despite the record of historical atrocities that any reasonably aware person could list, I think different cultures have evolved different levels of tolerance, acceptance, or celebration of violence.
A Spartan from 500 BC would be more likely to embrace violence than, say, a Hopi from the 19th Century.
So too, for that matter, chimp society is far more violent than bonobo society, and I don't think it's entirely clear how much that reflects a divergence in their evolution as opposed to different environmental pressures leading to different patterns and frequencies of aggression.
For example, male chimps are known to form gangs and raid other groups and terrorize females, and a male chimp may kill the offspring of another male if given the opportunity.
This doesn't happen with bonobos, where females group together and where, thanks to the legendary promiscuity of the species, males simply don't know whose offspring they would be killing assuming they had the chance.
It's difficult to say how much these difference reflect environmental pressures. One difference I've seen noted in the environments of chimps and bonobos that have been studied is the relatively more food-rich environment of the bonobo and the lack of competition with gorillas. This allows, the theory goes, females to group together and stay together as they feed.
Human genius and hubris lies in our creation of an environment and cognitive framework through culture. Irrespective of whether we're biologically more peaceful or less peaceful than our ancestors, the distribution of violence in the modern world reflects these factors just as the distribution of violence in another species may reflect, for example, population pressure.
In modern, post-industrial societies, violence seems to be highly stratified, with much of the population insulated from direct experience of it, and with the other fraction equipped to inflict disproportionate death. (One reasons why terror is an effective strategy?)
Thanks for sharing your thoughts.