Animals have speed and teeth. Humans have intelligence and the advantage of shaping the world to our vision. It's our ability to outmaneuver, to persuade, to seduce that puts us on top. We are animals to the very core with a goal of survival and propagation through control and dominance. Take away the pretty wrapping and mind games necessary to dominate in our modern society and you have the sheer joy of crushing an opponent, mind and body, to nothing-- the Stanford Prison Experiment, Rawanda, concentration camps. "Evil" is nothing more than the last resort of a survival instinct-- a lashing out from fear of being snuffed out first.
Throw some genuine mental illness into the mix and you have quite a bit to misunderstand and fear about "evil". If there is a spiritual evil present in the universe, it's a necessary player and shouldn't be given such a bad wrap.