Hurting Our Own Kind

Recently in my psychology class we have been discussing mutilation of ones own kind. We all know that some animals eat one another simply to live and survive. But why do humans kill other humans? You don'y usually hear about us eating eachother, so what is the reason? If revenge and anger and maybe sometimes to protect ourselves is the only answer, I find it shocking. Reflecting back on past events our society has always been like this. World Wars, Holocaust, Jonestown, and many others. Why do we do this and what drives up to that point.