Worst part of human nature - the application of intelligence to taking pleasure in the pain of others, also known as evil, deliberately prolonging and enhancing it. The ability to apply competition to such harm, like the Japanese soldiers having competitions to see who could rape the most women in a day during the Rape of Nanking, is parallel to this concept.
When a chimp wants you out of its territory and/or dead, it rips your arm off. It doesn't slowly rip off fingers, then break a wrist, then cut you to let you bleed to death to prolong the pain because that can give it pleasure.
That is why humans are unique in being capable of evil.
On the flipside, the ability to rise above our standard existence and engage in charity for the sake of strangers, risk our lives for those who aren't part of our "tribe", is why we can be good when animals run only on instinct.