I don't think power necessarily corrupts, but I do think a good chunk of what we consider 'moral behavior' is the result of the fact that if we do not behave that way somebody will hurt us or take away our privileges. We can not exercise power over anybody we want because they have power to strike back us. Any exercise of power against somebody who has no power to defend themselves from it is an offensive act. If Riker had the power of the Q he'd apply his own values to them, and in doing so he would be forcing his will on other people.
Your response to that might be, what if he only uses it defensively, like to save that little girl's life? In the insular case, if it's only that little girl's life, there's no reason not to. What if he made a rule of that behavior, saving anybody whose life was ever in danger? How does he decide who gets to be saved and who doesn't? Unless he does so with everybody and effectively negates mortality for the entire universe, he will be applying them selectively to the people he values more, creating a major advantage for people who go along with his own values over those who do not.
One person with absolute power is simply not capable of deciding what's best for everyone. Otherwise Communism would work.
I would even say, the only way to morally use the powers of the Q is to lose all emotional attachment to mortals.