You either die a hero, or you live long enough to see yourself become the villain

A quote from Christopher Nolan's The Dark Knight by the character Harvey Dent in reference to Julius Caesar and Batman. Repeated by Batman at the end of the film.

There are several possible meanings:

1. Absolute power eventually corrupts (as in the case of Julius Caesar).
2. Being consistently heroic is difficult if not impossible, circumstances will eventually push you towards villainy (as in the case of Harvey Dent and Batman).
3. Public opinion can easily change and will eventually change. Many people have the tendency find faults even when there are none. It can also be easily manipulated. You might adhere to your code despite 1. and 2., you can still be painted as a villain by your enemies (as in the case of what Joker did to Batman).
4. Villainy and Heroism are relative and not absolute. "Good" will change with time. Different people have different concepts of "good" (the actions of Julius Caesar can be judged very differently by different people throughout the different ages). In many situations, the line between the two concepts can be very blurry (following the law versus vigilantism).