Edward…
It is with deepest regret that I found myself in a predicament involving, of course, you. What else would it be? I had to make a split-second decision, and my choices were to either tell you the truth (as much of it as I've uncovered, at least), or to deceive you as well as the brass. If I told you the truth, you may have come under fire should my trickery be revealed, and you'd also be involved further in this mess than you already are. And if I were to lie to you…well, it's not a white lie, that's for sure. I knew what consequences this choice would bring, and still I had to make it.
Better for you to hate me until I know it is safe to tell you the actuality of the situation, than for me to risk your life and career as well as my own.
We are playing a dangerous game, you and I, although I don't know if you realize quite how high the stakes might be. I've always been ambitious, but I've also always harbored a bit of a rebellious streak, and so it's nothing much for me to take risky action. I don't mind taking on an insurmountable foe, because if I succeed, I will have gained much, and if I fail, I will most likely perish. With my own life, I am wiling to make that gamble, but with yours I am not.
I did not murder Ross, as you believe. I've in fact ferried her away to safety. I could not let an innocent die, but I also must play the part of obedient Colonel until I know just what powers that be have been corrupted.
As of this moment, I believe my story has been accepted. Once I'm assured of this, I have every intention of telling you the truth. For one thing, though I am loathe to involve you in something of this magnitude, you've already involved yourself and so it is a moot point. Furthermore (and oh, do I hate to admit this!) I simply cannot accept a future in which you despise me so.
Don't worry, Ed; your time is coming. One day you will understand how a chess master must play, thinking moves ahead rather than acting rashly.
Yours,
Roy Mustang