This particular subject has always been close to my heart. How does a 'hero' become a 'villain'?

Perhaps further, what is a hero? what is a villain? And what makes them so? Good and Evil are, ultimately, subjective, either the opinion of the individual, by their conscience, or dictated by a higher power, be that higher power a government or a god. What happens when the conscious of the individual clashes with those of the high power? In that situation, how does one know which is right? 

All fun philosophical fodder for a story. Very easily can the line between villain and hero be blurred beyond all recognition.

I'm going to do my best to get a submission in for this one. That said, I'm also a legendary procrastinator, so take the previous statement with a grain of salt.