They have a complete lack of empathy, which probably comes from their limited capacity for abstract thought in general. Also they don't internalise morality like whites do, it must be forced upon them by a "big man" authority figure or the mob, so going out of their way to kill people in the slow / painful / humiliating ways helps to maintain order.
https://whitelocust.wordpress.com/morality-and-abstract-thinking-how-africans-may-differ-from-westerners/ 