Speaking not of politicians, but of everyday situations. You talk mean about someone behind their back, because you CAN do it. You boss over someone and make them feel small because you can do it, knowing its wrong, just because you have the power to do it. You talk rudely to your parents or family, because you know you can get away with it.

If on such microscopic levels power can corrupt us, why do we blame politicians when they get corrupt.

In corollary, an evolutionary consequence is that the feeling of guilt is slowly being replaced with the fear of being found out.