Being almost people, it's no surprise chimps regularly kill other chimps for territory and natural resources, with entire chimp tribes going at it. But to make it onto this list, they had to do something that goes beyond regular animal savagery and right into horror movie stuff. The horror movie in question is of course Italian classic, Cannibal Holocaust.
Chimps, you see, have a tendency to suddenly grab a baby chimp from its mother's arms and chow down, sharing the meal with the entire group. Well, except for the mom, who presumably prefers to stay in a corner screaming: "Holy shit! Holy shit!"