I think a really important point here is that power amplifies the person. It gives already existing personality dispositions and tendencies a louder voice, and increases the chances that these tendencies will be given fuller expression.

Most people don't live by a logically consistent framework, or even contemplate what it means to not be corrupt, much less what it takes to be a 'good person'. Power merely amplifies your flaws here. A good pair of examples of this is Lincoln and Ron Paul. Just look at both their records in the Senate: Lincoln supported the same crony BS he put into 'maximum go' as president. Ron Paul, in contrast, was Dr. No.

In any event, this is why Gandalf rejected the Ring: Though he had strong convictions and mindfulness about what power can do, there's always the doubt that something about your worldview could be weak enough that amplifying that with power would cause disaster from.