Power is the ability to reward and punish others while being relatively unaffected by their attempts to reward or punish you. Power corrupts, which is another way of saying that having power makes it easier to lie, cheat, steal, inflict pain, or otherwise engage in bad behavior. This is partly because power tends to dampen fellow-feeling but also because powerful people tend to be strongly goal-directed - that is, focused and purposeful. And, well, sometimes one has to step over people to get to where one’s going. Or break some rules.
Don't get me wrong - in general, being focused on achieving goals is a good thing. Strong goal direction is associated with success in school, work, and relationships. But when goals become the be-all/end-all, bad behavior ensues as the ends come to justify the means and whatever doesn't serve the goal is simply ignored.
So to the powerful, I say: stop, breathe, and try to be nice, even if you don't have to be.
References:
Dubois, D., Rucker, D. D., & Galinsky, A. D. (2015, January 26). Social Class, Power, and Selfishness: When and Why Upper and Lower Class Individuals Behave Unethically. Journal of Personality and Social Psychology. Advance online publication. http://dx.doi.org/10.1037/pspi0000008
Ordóñez, L. D., Schweitzer, M. E., Galinsky, A. D., & Bazerman, M. H. (2009). Goals gone wild: The systematic side effects of overprescribing goal setting. The Academy of Management Perspectives, 23, 6-16. http://journals.aomonline.org/amp