Out of all the mammals on Earth, only chimpanzees and humans tend to enact lethal violence upon members of their own species. Other mammals may display aggression or violence in order to compete for food, territory or mates, but rarely act so violently as to kill their competitors.

Scientists have observed chimpanzees forming raiding parties that attack stragglers from neighboring groups of chimps. These attacks are vicious, with several chimps from the raiding party attacking a single victim. Renowned primatologist Dr. Jane Goodall witnessed a battle for territory among chimps that lasted four years. In the end, one group of chimps killed every chimp in the opposing group. Male chimps have also been known to commit infanticide, though its not definitively clear why the infant chimps are killed [source: Jane Goodall Institute of Canada].

A 10-year study completed in 2010 tried to explain the chimps' violent group behavior. The study of chimps in Uganda by a University of Michigan primate expert determined that group violence between chimpanzees had to do with groups expanding their territory at the expense, and lives, of other chimps that stood in the way. Once the other chimps were dead, the expansionist group moved into the territory and began to enjoy its resources, such as its fruits and the new, wider area in which to socialize [source: physorg].

But as a species, it's hard to beat human beings on the "most violent" scale. While we are capable of using reason and weighing the consequences of our actions -- something a good many people do throughout their lives -- we're also able to design effective ways to hurt seriously or even kill each other. From primitive stone tools adapted into weapons to powerful nuclear bombs, we've turned violence into a science. Our species is responsible for the extinction of thousands of other species, and we're no slouches at killing each other off. One need only look to some of history's most "successful" mass murderers for evidence of that. Monsters such as Josef Stalin, Adolf Hitler, Mao Tse-Tung and Pol Pot are among the prime exemplars of violence within a species.