Human Beings are inherently NON-violent towards other Human Beings.
I believe that the truth of human behaviours and motivations is never this simple. I do agree that there is a significant amount of truth in this statement though, in terms of how often this rule of thumb can be applied. With one proviso - The question of who an individual defines as a "human being".
Once you dehumanise a group of people, it becomes a great deal easier to kill them. My understanding is that a large part of what happens to those involved in war, whether this is created intentionally by those in command, or as a coping mechanism, is just that.
The only thing we have approaching an example of raw 'human nature' (a concept I think is problematic at best) are tribal hunter gatherer cultures. I seem to recall several examples where the name for the individual's tribe is something like 'the people' or whatever. I would leave this line of argument for someone who actually knows something about this though.
Also worth noting - I have heard of tribal law with provisions for dealing with people who killed within the tribe. Where did this come from? Again... if anyone is an anthropologist - I'm sure they can take this further than I can as I could well be talking crap.