Here's another reading: It could also be that we're wired to exploit situations of power. Isn't it convenient that empathy is reduced when you're more powerful, making it more likely for you to (ab)use that power in your own interest? That seems awfully likely to improve your personal reproductive success...
Indeed the sociology research that I've seen on the latter question says that leaders who act in an entitled amoral fashion lead fundamentally less effective organizations than leaders who encourage empathy and cooperation.
Group behavior is merely an evolutionary strategy. And interesting group behaviors can only be understood by analyzing how individuals in the group are incentivized.
maybe powerful leaders who happened to be ruthless spread their genes ruthlessly, more so than the powerful and benevolent
edit: ugh, grammar. this is what I get for posting from a phone