Disclaimer: Not mine, yo.
He almost writes the letter three times.
Regulus knows he's not going to live much longer. He's turned against Voldemort, betrayed him, and the Dark Lord has ways of ferreting out traitors and doubters. Some say the man - if he can be called a man - reads minds.
Regulus doesn't believe that, though. After all, he was able to steal Slytherin's locket with no one the wiser.
I'm not sure what to say to you, brother. We've never been close – you the prodigal son who never returned home, me the golden Black child. All I can think to say is…you were right. About Voldemort, about the Death Eaters.
But you were wrong about me.
Of course, stealing one of Voldemort's horcruxes was pretty much the equivalent of jumping off a cliff without a magic carpet or broom to catch his fall. There's no going back, not unless he learns to fly – and the chances of that happening are as nonexistent as his so-called master's mercy.
I was blind, Sirius, and stupid, but I've opened my eyes and trust me when I say my brain's finally working. You were right, and I'm too much a Black to remain willfully in the dark any longer. We may be heirs to the House of Arrogance, you and I, and we may hate admitting we're anything less than perfect, but I think we both hate being wrong even more.
Every hour brings him closer to oblivion, death. And he really ought to be out, making the most of these hours, but instead he's pacing his room, composing a letter in his head that he knows he's too much a coward to set to paper and ink.
He can face ugly truths and imminent death, but Sirius….
Sirius is another matter entirely. He always has been.
I've done what I can. It's not much, but it's more than most could do. I've stolen something from him, something important to him, something he needs to survive. If I could destroy it – but I don't know how.
I bet you never thought you'd wish Mother and Father had taught us even more of the Dark Arts – but if they had, I could have struck a blow against Voldemort unlike any injury he's yet suffered. If I just knew how to destroy it….
But I don't. Ignorance is failure, I'm afraid.
Regulus wonders what Sirius would do with the truth, if he could accept that his younger brother died doing the right thing. There's been so much…so much hatred, so much resentment, so much bitterness between them – he almost thinks the truth would be too much for his older brother.
And he fears that it wouldn't be enough.
I'm going to die, Sirius. He's going to kill me, or one of his servants will. Either way, I'm going to die, and soon.
The plain truth of the matter is, he thinks it might be better if Sirius believes the lie – that his brother died a coward, a Death Eater who got in a little too deep and panicked. Better he believes the lie, and despises him and pities him in peace.
It would be infinitely worse for Sirius to know the truth of his death and still scorn him for not seeing the reality of Voldemort's plans sooner, for not doing enough to foil the Dark Lord, for not being Black enough or hero enough or good enough.
Odd, how suddenly his brother's opinion matters so much more than anyone else's.
I don't want to die, Sirius. I'm not like you. I don't court danger and mayhem. I'm supposed to be the reliable one, the dutiful son, not the adventurer.
I hate Voldemort for doing this to me. And I hate Mother and Father for making me too much a Black to remain his slave.
I don't want to die, but I will. Will you even mourn me?
And every time he almost writes the letter, that's where he stops. Because he doesn't want to know, doesn't want to wonder, if Sirius even cares.
And he doesn't want to know what else he'd compose in his letter, if he keeps going. Bad enough that he open himself so much, as it is.
Three times, he almost writes the letter.
But in the end, he prefers the lie. The truth's already ruined him.