I would also agree that morality is highly dependant on one's environment. If one had to survive in a world where virtually all people from time to time commited murder (something possibly true in pre-historic times) he would have no real incentive to act against such a capability for violence in himself either. On the contrary one would feel left out if he in his view expressed more hostility than the average person in his environment did (since this could lead to him being marginalised, either by others, or through reflection on his supposed differences from them in regards to basic morality).
Morality, when not taken to extremes, only serves the basic ability to socialise.