Several years ago, in a village of China, a guy killed 14 corrupted government officers, and was sentenced to death in the end. Before he was executed, he claimed that he did that for justice and the regret is that he couldn't kill more who deserved the death. Several days ago, the war between Russia and Georgia erupted. Thousands of people died as in many other military conflict. And this is even a small number compared with 1.3 million people died in Iraq war.
No matter big or small, are these things that we are trying to avoid all the time? Are this done just by some "evil" country/government? We don't know how future goes but maybe we can get some clue from history. WWI finished, WWII continued; WWII finished, Code War started; Code War ended, Iraq war broke; etc.. It seems that killing never ends. People all suffered from all those things year by year. Governments all over the world accused each other for doing killing but it is the altitude only to others but for themselves. The war proceed any way no matter it is protested or supported. Usually people commit to the war in the name of "nice" reasons.
What is the power behind these social phenomena? Of course every thing when goes to the most fundamental principle is about physics laws, or we can say God here. As a human ourselves, we may ask what the reason in the next understanding level (social science)? As we know our ancestors has been through a long history of evolution, fighting against the environment, other species, our own species. Probably we've gotten some in our soul that we want to fight and live better than others. So killing or massacre is just a thing that we cannot neglect. In another words it is like the moon goes around the Earth. It is the law.