Disclaimer: Harry Potter is not mine.
Nothing is clear in his head. In fact, there is nothing much inside of his head other than the strange buzzing noise that seems to be drowning out his ability to think. He remembers being angry. He remembers being afraid. He remembers panic and desperation. He remembers feeling as though he was moving too slowly and then everything feeling frozen in time all around him. Those emotions are swirling around in the background somewhere beyond the buzzing, but he cannot think why. Everything feels wrong. He is wrong. He was wrong. Everything has gone wrong because he was wrong. He thinks that he is laughing.
It is detached and far away, but he can hear the sound of it (and he is almost certain that he is the one that is making that sound). It is not an amused chuckle or the gasping for breath moment he has experienced at times before his world came crashing down around his ears. It is a harsh, barking sort of a sound, but it is laughter just the same. It is a sound of brokenness. It is the sound of someone who has lost. He lost something or someone. Something is lost. He is the one who lost it. Any thoughts beyond that get drowned out by the buzzing and are pulled away from him before he can manage to catch hold or force them to become clearer.
He wants something that he cannot have - something that he can never have again. He thinks that he might be mourning. Mostly, he is certain that he has been a fool. He has always thought himself rather clever. He is certain in this moment that he is not. He has been foolish, very foolish. It was something to do with trust and friendship and lies, but he loses even the pretext of thinking next as the buzzing gives way to darkness.
When the darkness recedes, he is first aware only of being numb. He is being led somewhere by someone (he thinks a few someones), but he feels too withdrawn within himself to make the effort to determine whom it may be or where they may be taking him.
It is then that he remembers why it is that he feels the way that he does. James is gone. Lily is gone. They have been murdered in their supposed to be safe home. They trusted him, and he let them down. He has killed them by proxy. He tries to clear his mind enough to remember what has happened to the baby. He thinks . . . no, he knows that Harry survived. He remembers seeing him. He heard his crying and saw the wound across his forehead. He has shattered Harry's world - lost him his parents and destroyed his home. What will happen to the little boy now?
He was the godfather. Harry was to be his responsibility only that was never supposed to be. James and Lily were not supposed to go away. They were supposed to remain responsible for Harry. It was never really supposed to be Sirius's job to do. He does not know where it is that he is going, but he is certain that the people surrounding him are not taking him to Harry. It may be just as well. He has been the cause of enough destruction already.
He notices the cold seeping into his limbs and finds it to be appropriate. It seems like something that should be his new normal. It should be cold. The world should feel strange. Nothing should feel like the old normal ever again because the old normal will never be again. He was wrong. He was so very wrong, and it seems a very unfair thing to him that the members of the little family from Godric's Hollow are the ones who have to suffer for his mistakes.
He realizes that he is sitting down on a cot of some sort only moments before he realizes that he is now alone. The people that were leading him have disappeared while he was not paying attention. He has the sudden conviction that he has not been paying attention to anything outside of himself for some bit of time. He blinks, and his thoughts grow clearer and less internally focused. He attempts to determine where exactly it is that they have placed him.
He decides that it is a cell of some sort as he studies the thin blanket over the not much thicker mattress that make up the accessories of the cot upon which he is resting. There is little else to see besides the four walls. There is a barred over window, but he is feeling strangely exhausted and cannot summon enough curiosity to go and look out of it. He wonders if he has gone completely crazy with grief and anger and been placed in some secure ward of St. M's that he has never seen before.
That is when the voices start. He recognizes each and every one of them - he knows them from his memories and even more clearly from his nightmares. He knows where he is. It occurs to him then that there are only two people left in the world that know the truth about what he did - that he caused James's and Lily's deaths but not in the way that everyone will think that he did. They haven't asked him to explain (or maybe they did and he was too far inside himself to realize what they were doing).
It plays over and over again in his head - the memory of insisting that they switch to Peter. He is confronted with the knowledge of his arrogance - the way that he was so certain that he had it all figured out and calculated and knew what was best for everyone. He watches the trust in Lily's eyes and remembers the smile of recognition with arms reaching out that he used to receive from Harry. He hears James tell him that they will do it his way, and he barely reaches the corner in time to retch with the knowledge that he is responsible for the way that it all came undone. He sees the flicker of hurt in Remus's eyes as he cold shouldered him at their last meeting and wishes for nothing more than the chance to tell them all how sorry that he is that he couldn't get this right.
Then, the sadness that overwhelms when the nasty creatures congregate outside his door is replaced with something different. He knows that he was the cause, and he knows that there is no way for him to make amends. He cannot fix it, but he can cause retribution. He is still sad and tormented, but those both drown in the sea of rage that begins to take over his every waking moment.
Then, he realizes what he must do. It is the only thing that he can offer to the rest of them. He does not know how or when he will be able to take action, but he will see it happen. Peter must die.