How can that happen, you ask? How Can Someone be so pure? so honest? so noble? How Can someone strive to be virtuous? How Can someone as powerful as Superman not abuse his power or even be tempted to do so?
I think the better question is ''Why do you expect power to be, in and of itself, something that is so tempting and corruptible?"
The key is really what is your purpose and perspective in life? Are you selfish? Looking to help only yourself? Or are you selfless or just plain helpful, such that anything you have, or Can do, you use in a way to be of benefit to others, rather than simply yourself?
That is the big difference between fans of Superman: The Movie and fans of the Man of Steel, do you approach the world as helping yourself first or are you helping everyone equally?