Dear Sirius,
Where to start? There's no place like the beginning of it all, I suppose. I'll be blunt, brother, I hated you for leaving 12 Grimmauld Place five years ago. Nevertheless, it was a hatred not nearly as virulent as the one for my own solidarity. Whatever Mother had you believe, you were always a determinate force in my life. What they called your "indignant liberal rhetoric," I believed. I always believed you, Sirius; I never, however, articulated my beliefs, and for it, I am so entirely ashamed. Not just because I couldn't stand up for myself, like you were always telling me to do, but because once I didn't speak those ideas, they got pushed back in my mind. I began to tell myself that certain things were okay, that although some ideas overlapped and contradicted others, they were still inherently moral and virtuous. I became a master at doublethink.
I'd give up all the prestige of the Most Ancient and Noble House of Black a thousand times to change it, but unfortunately, the choice was already made. My hands are already stained with the blood of the innocent, no absolution in sight. Sometimes, I wonder if it would have been different, if we could have been different, if I would have listened to you trying to give me the beginnings of a backbone. I don't know if you'll get this, and even if I were certain, I wouldn't be able to disclose too much information, especially regarding my whereabouts. I often wonder whether I'm doing this for my own conscience, or for the distant glimmer of hope I have that you'll find out and recognise me for a lesser coward. But brother, I assure you, if you could know what I'm about to do, you would be so proud. I used to be so frightened of the consequences, the consequences of righteousness and all it entailed. I'm not afraid anymore, Sirius. Death is but a new beginning.
In some ways, what I'm doing is, essentially, signing my death warrants. I'm in so deep that my demise will go unnoticed, or at least unspoken. And Sirius, it go so dark, so deep. When our childhood duels became battles of life and death, when He asked me to betray the ones I loved, to betray you, I knew that something had changed. I could no longer rely on the comfort of living in your shadow. I was setting my own path for myself–Slytherin ambition never failed me–but in another's shadow, in the Dark Lord's shadow, and it was a shadow cast that I was not too certain I could stand behind.
Oh Sirius, I wish I could tell you, I wish I could tell you all those things not even I'm supposed to know. I want to help you and your friends, I want to prove to you that I can be everything you thought I could be one time so long ago. But I know this is the only way. And I wish I could give you more than that, but considering the fact that this may not even reach you, to which extent I am merely rambling for my own sake, I cannot.
But brother, what I'm about to do, and the martyrdom–my martyrdom–it requires is going to help in the long run. Because I won't fail you, not this time.
Yours,
R.A.B.
P.S. Keep an eye out for that Potter child when I'm gone, will you? I know he isn't due for some months now, but trust me, it's important.