Most people have heard the line "Power corrupts." (Or the longer version, "Absolute power corrupts absolutely.". The question I'm often asked is "why and how does power corrupt leaders?"
The answer is complex, but fairly clear. Leadership, at its core, is all about power and influence. Leaders use their power to get things done. A simple distinction is between two forms of power. Socialized power is power used to benefit others. We hope that our elected officials have this sort of power in mind and are primarily concerned with the best interests of their constituents.
The other form of power is called personalized power, and it is using power for personal gain. Importantly, these two forms of power are not mutually exclusive. A leader can use his or her power to benefit others, but can also gain personally (there are no poor former U.S. Presidents!). The obvious problem is when personalized power dominates and the leader gains, often at the followers' expense.
Yet, leaders can delude themselves that they are working for the greater good (using socialized power), but engage in behavior that is morally wrong. A sense of power can cause a leader to engage in what leadership ethicist, Terry Price, calls "exception making" - believing that the rules that govern what is right and what is wrong does not apply to the powerful leader "for other people, this would be wrong, but because I have the best interests of my followers at heart, it's ok for me to...." During Watergate, the argument was made that President Nixon could not have acted illegally because "the President is above the law."