Competition
Human beings, as the most dominant species, face the most cruel intraspecies competition. The following passage describes one such competition.
David and his men spent their time raiding the Geshurites, the Girzites, and the Amalekites—people who had lived near Shur, toward the land of Egypt, since ancient times. David did not leave one person alive in the villages he attacked. (1 Samuel, Chap 27)
While not all competition is as direct, no competition is less cruel. Indeed, few think David is a cruel man. David is one of the most beloved men in human history.