I think if people were willing to be violent within their in-group merely because of hunger, all of human history would have gone differently.

I am not sure Maslow is saying what you think he is saying, since after all he grew up in Brooklyn. :-)

I would suggest reading Meditations on Violence, which is a beautifully flawed book, including a great section outlining a theory of violence, and what precipitates different types of violent behavior.

http://www.amazon.com/Meditations-Violence-Comparison-Martia...