Communal sadism and the inability to recognize and control one's own primal urges. The outlet does not tame those urges but driving them toward their manifestations, giving form of those emotions and thoughts into actions. Warfare is the most extreme display of such human nature, but things like sports and fight for social status are also just different sides of the same coin. There is a very fine line between engaging in those activities for the sake of passive enjoyment and the desire to make winners and losers. Just look at competitive video games. The only healthy way to engage in those activities is when you have no desire to win whatsoever. The moment you care about winning, it becomes merely a means to an end.
George Orwell in The Sporting Spirit disagreed with the take that sports are a force for good. He saw them as gross manifestations of nationalism.