Assertion:
"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 holders of power, increasing as the power increases."
Rationale:
"Historic responsibility [that is, the later judgment of historians] has to make up for the want of legal responsibility [that is, legal consequences during the rulers' lifetimes]. Power tends to corrupt and absolute power corrupts absolutely."
He then goes on to rail against people that hold power:
"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."
Without the idea that power tends to corrupt those in power, there would be no reason to presume that they did wrong by default or to say that people who exercise influence and authority are bad men.