“What is true is that the idea of power corrupts. Power corrupts most rapidly those who believe in it, and it is they who will want it most. Obviously, our democratic system tends to give power to those who hunger for it and gives every opportunity to those who don’t want power to avoid getting it. Not a very satisfactory arrangement if power corrupts those who believe in it and want it.”

Nadia Chernyshevski
Green Mars (1993)

Source: Becoming a Person of Influence: How to Positively Impact the Lives of Others

Acceptance Speech http://www.washingtonpost.com/wp-srv/politics/daily/may98/goldwaterspeech.htm as the Republican Presidential candidate, San Francisco (July 1964)
Unsourced variant: Now those who seek absolute power, even though they seek it to do what they regard as good, are simply demanding the right to enforce their own version of heaven on earth, and let me remind you they are the very ones who always create the most hellish tyranny.
Context: Those who seek absolute power, even though they seek it to do what they regard as good, are simply demanding the right to enforce their own version of heaven on earth. And let me remind you, they are the very ones who always create the most hellish tyrannies. Absolute power does corrupt, and those who seek it must be suspect and must be opposed. Their mistaken course stems from false notions of equality, ladies and gentlemen. Equality, rightly understood, as our founding fathers understood it, leads to liberty and to the emancipation of creative differences. Wrongly understood, as it has been so tragically in our time, it leads first to conformity and then to despotism.

Letter http://oll.libertyfund.org/index.php?option=com_content&task=view&id=1407&Itemid=283 to Mandell Creighton (5 April 1887), published in Historical Essays and Studies, by John Emerich Edward Dalberg-Acton (1907), edited by John Neville Figgis and Reginald Vere Laurence, Appendix, p. 504; also in Essays on Freedom and Power (1972)
Paraphrased variant: All power tends to corrupt and absolute power corrupts absolutely.
Context: I cannot accept your canon that we are to judge Pope and King unlike other men, with a favorable presumption that they did no wrong. If there is any presumption it is the other way against holders of power, increasing as the power increases. Historic responsibility has to make up for the want of legal responsibility. Power tends to corrupt and absolute power corrupts absolutely. Great men are almost always bad men, even when they exercise influence and not authority: still more when you superadd the tendency or the certainty of corruption by authority. There is no worse heresy than that the office sanctifies the holder of it. That is the point at which the negation of Catholicism and the negation of Liberalism meet and keep high festival, and the end learns to justify the means.

Source: Academy Series - Priscilla "Hutch" Hutchins, The Engines of God (1994), Chapter 25 (p. 356)

Source: Chapterhouse: Dune

2012, Yangon University Speech (November 2012)

Free speech in an age of identity politics (2015)
Context: To accept that certain things cannot be said is to accept that certain forms of power cannot be challenged.... This is why free speech is essential not simply to the practice of democracy, but to the aspirations of those groups who may have been failed by the formal democratic processes; to those whose voices may have been silenced by racism, for instance. The real value of free speech, in other words, is not to those who possess power, but to those who want to challenge them. And the real value of censorship is to those who do not wish their authority to be challenged. The right to ‘subject each others’ fundamental beliefs to criticism’ is the bedrock of an open, diverse society. Once we give up such a right in the name of ‘tolerance’ or ‘respect’, we constrain our ability to challenge those in power, and therefore to challenge injustice.