You were not a man in the habit of questioning himself. For all 29 years of your life, you have been, in the words of Lieutenant Colonel Hughes, 'A self-righteous bastard who never lets anything get in his way'. You have never felt lost. When you were a child, whatever you wanted, you took—and when you couldn't just take it, you whined, wheedled, and pestered Mother until she got it for you. Certainly, you must have been an odd child. The first time your mother asked you what you wanted to be, you answered, 'King of the whole world!'
Alchemy had opened up another world to you at the age of 13. You liked the idea that alchemy was a manipulation of inherent power. If Edward Elric's undying alchemic principle was 'equivalent trade', yours was 'manipulation to change'. You liked the idea of changing the state of things subtly to great result, and you were a master at it. You knew you were a master at it, had always known it. To know that even the elements were not immune to this was exhilarating to you.
You were going to change everything that way. The world would bend to the will of Roy Mustang, Generalissimo. You joined the army because you knew that you'd have to get there, first; but that was no deterrent. You would rise to the top by hook or by crook, or if it was necessary, by flame, because that's where you belonged—in charge.
But then you went to went to Ishbal, and in a night, everything changed.
There are times when all the manipulation in the world cannot help you. Cannot save you. Nothing can bring back the dead, or call back the bullets in the bodies of the Rockwells. Bullets you fired. Deaths you caused.
For a moment you told yourself it was an order, one that could not be manuvered around, but it is a flimsy wall that collapses in on itself without scrutiny. And for a moment, your life, with all its high aspirations, was worth less than the dust on your boots.
You would have pulled that trigger, if Dr. Marco had not been there.
Your self-confidence had crumbled. Your purpose was lost. What place for a murderer, for a mindless follower of criminal orders? None in this world. What right did you have to change everything? What would you change it to?
With six words, Dr. Marco walked away, and you had your answer.
Now, you are still a self-righteous bastard who won't let anything get in his way. Perhaps you are even more of one now then you once were.
You're going to change everything.