This seems to stem from the idea that you have to be a ruthless monster in order to achieve power in the first place. The truth might be scarier. For years now, several groups of scientists have been studying the ways power impacts the human brain. They’ve found that power causes people to become more impulsive, less conscious of risk, and less able to empathize with others. The effects are severe enough to be comparable to brain damage.

(From http://www.cracked.com/blog/5-reasons-horrible-dictators-always-catch-us-off-guard/)