So does power always corrupt? Just as in life, there is no clear answer in the movie. It can do both.
The phrase "Power tends to corrupt, and absolute power corrupts absolutely" is often quoted on its own. But the second part is less well known: "Great men are almost always bad men". (Attributed to John Emerich Edward Dalberg Acton, first Baron Acton: with more information here) The second part could well be a nod towards believers in an omnipotent God who might challenge the idea of absolute power corrupting all beings absolutely.