I agree that statements like all As are Bs are likely to be only approximately true and if you look you will find counter examples. But... 'power corrupts' is a fairly reliable rule of thumb as rules of thumb go. I include a couple of refs that took all of 3 minutes to find although I couldn't find the really good one that I noticed a year or so ago.

http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1298606 abstract: We investigate the effect of power differences and associated expectations in social decision-making. Using a modified ultimatum game, we show that allocators lower their offers to recipients when the power difference shifts in favor of the allocator. Remarkably, however, when recipients are completely powerless, offers increase. This effect is mediated by a change in framing of the situation: when the opponent is without power, feelings of social responsibility are evoked. On the recipient side, we show that recipients do not anticipate these higher outcomes resulting from powerlessness. They prefer more power over less, expecting higher outcomes when they are more powerful, especially when less power entails powerlessness. Results are discussed in relation to empathy gaps and social responsibility.

http://scienceblogs.com/cortex/2010/01/power.php from J Lehrer's comments: The scientists argue that power is corrupting because it leads to moral hypocrisy. Although we almost always know what the right thing to do is - cheating at dice is a sin - power makes it easier to justify the wrongdoing, as we rationalize away our moral mistake.