Not only does Acton say that power corrupts men, he says that corrupt men get power and that "great men are almost always bad men."
First, in an 1881 letter to Mary Gladstone, daughter and secretary to the British PM, Acton wrote:
And remember, where you have a concentration of power in a few hands, all too frequently men with the mentality of gangsters get control. History has proven that all power corrupts; absolute power corrupts absolutely....The danger is not that a particular class is unfit to govern. Every class is unfit to govern."
And in an 1887 letter written to the Anglican Bishop of London, Mandell Creighton, Acton (a Roman Catholic) makes reference to the papacy.
"I cannot accept your canon that we are to judge Pope and King unlike other men with a favourable presumption that they did no wrong. If there is any presumption, it is the other way, against the 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."