This internal transformation is mirrored on a macroscopic scale by the movie's premise. It was the quest for a utopia - specifically, a world free of violence and aggression - that led to the invention of the aptly-named drug Pax. The drug caused most people to lose the will to live - while in a small minority, it had an opposite, and even worse, effect. Mal must force his crew, in the most horrifying way imaginable, to mingle with that banished, evil side.
And come to think of it, isn't it strange how River's name sounds so much like the word "Reaver"?
Aggression and conflict are part of our reality. There is conflict in our world, there is aggression in our nature. We don't get a choice in this. What we do get to choose is how to handle these things. We can run from them and drive them underground; or we can acknowledge them and work with them responsibly.
If we refuse to acknowledge the dark side of ourselves, we are only inviting it to do us harm. To embrace it is to learn true power, and humility, and wholeness.