“It was not power that corrupted people, but fools who corrupted power.”

Source: Freedom from Fear (1991)
Context: It is not power that corrupts but fear. Fear of losing power corrupts those who wield it and fear of the scourge of power corrupts those who are subject to it. Most Burmese are familiar with the four a-gati, the four kinds of corruption. Chanda-gati, corruption induced by desire, is deviation from the right path in pursuit of bribes or for the sake of those one loves. Dosa-gati is taking the wrong path to spite those against whom one bears ill will, and moga-gati is aberration due to ignorance. But perhaps the worst of the four is bhaya-gati, for not only does bhaya, fear, stifle and slowly destroy all sense of right and wrong, it so often lies at the root of the other three kinds of corruption. Just as chanda-gati, when not the result of sheer avarice, can be caused by fear of want or fear of losing the goodwill of those one loves, so fear of being surpassed, humiliated or injured in some way can provide the impetus for ill will. And it would be difficult to dispel ignorance unless there is freedom to pursue the truth unfettered by fear. With so close a relationship between fear and corruption it is little wonder that in any society where fear is rife corruption in all forms becomes deeply entrenched.

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: General sources, Chapterhouse Dune (1985)
Context: All governments suffer a recurring problem: Power attracts pathological personalities. It is not that power corrupts but that it is magnetic to the corruptible. Such people have a tendency to become drunk on violence, a condition to which they are quickly addicted.

The Short Reign of Pippin IV (1957), p. 102

Source: Steps to an Ecology of Mind (1972), p. 494

Source: The Postman (1985), Section 3, “Cincinnatus”, Chapter 14 (p. 267)
Variant: It is said that power corrupts, but actually it’s more true that power attracts the corruptible. The sane are usually attracted by other things than power.
As quoted in Values of the Wise: Humanity's Highest Aspirations (2004) by Jason Merchey, p. 120
This is very similar to the expression by Frank Herbert in Chapterhouse: Dune (1985): "All governments suffer a recurring problem: Power attracts pathological personalities. It is not that power corrupts but that it is magnetic to the corruptible. Such people have a tendency to become drunk on violence, a condition to which they are quickly addicted."
Context: It’s said that “power corrupts,” but actually it’s more true that power attracts the corruptible. The sane are usually attracted by other things than power. When they do act, they think of it as service, which has limits. The tyrant, though, seeks mastery, for which he is insatiable, implacable.

The Corruptions of Our Time, p. 248
The Corrupt Society - From Ancient Greece To Present-Day America (1975)