The world is full of suffering. Humans seem particularly talented at inflicting suffering to others. We wage cruel wars, kill millions in gas chambers, burn so-called witches at the stake and so on. Think of the most cruel thing you could imagine and some person (typically male) will have committed it somewhere, someplace. Why do we commit these atrocities?
To answer this, let's make it less personal. A certain kind of parasitic wasp stings spiders into a sort of coma and lays her eggs inside it. By the time the eggs hatch the larvae eat the living spider inside out. An even more bizarre adaptation can be found in another kind of parasitic wasp, Glyptapanteles, who's larvae manipulate their host caterpillar into a bodyguard (see http://www.world-science.net/othernews/080605_glyptapanteles.) Pelicans usually hatch 3 eggs. In good times all three survive but usually one of the older brother/sister kicks the smallest baby pelican out of the nest which of course immediately dies. One more: when a lion becomes the new alpha-male of a group it will kill each and every infant lion to make sure it is his genes that get propagated into the next generation. Needless to say, this is but a small selection of a very large collection of examples of the cruelties one can find in the animal kingdom.
Back to humans. We evolved in social groups and have all sorts of fantastic social adaptations that let us function in groups. I have been told a very significant part of the brain is devoted to social interactions. Presumably, these groups were constantly in fierce competition. There may have been strong selection for the variety of proto-humans that were most effective in exterminating their competing neighbors. Peacemakers may simply not have had the edge in evolution. The recent genocide between Hutu's and Tutsi's may serve as a flashback to how things may have looked like in the stone-age (don't get me wrong: "we" -- white Caucasian males-- are no better.)
How deeply burried is the animal inside us. I believe it is not far away at all. What does someone have to do to you in order to fall back to your prehistoric self. How about someone killed your family? Personally, I cannot predict how I would react, but likely very aggressive and ready to march into war. Another unsettling example is the second world war. I grew up in the Netherlands and although some Dutch like to think of themselves as different from Germans, Dutch and Germans are virtually the same race. We all know what some Germans were capable of in WO-II, and I have no illusions about myself ending up fighting on the wrong side, given certain conditions were met (such as living in Germany in 1941, having patriotic parents and being influenced by the well oiled nazi propaganda machine etc.)
To study how easily people are manipulated into committing acts that border to torture Stanley Milgram set up some cunning experiments in 1961 (see http://www.world-science.net/othernews/080605_glyptapanteles.) He basically ordered students to administer electrical shocks to subjects that they could not see but could hear screaming. The pretend-professor only had encourage the students a bit and claim this was all for the good of science for these students to almost "kill" these pretend-subjects. Of course, there is nothing special about Yale students, and we should have no illusions that we can be tricked into similar acts.
I think we should admit to ourselves that a part of us is still governed by animal drives and instincts, perhaps inconsciously. This is what creates a lot of the suffering we see even today. We should guard ourselves against it by at least recognizing it, and hopefully one day taming this beast.