Face the music.
SIRIUS
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After James left to find Snape and Remus, Peter punched him in the face so hard he was knocked to the ground.
“Pete! What the fuck?” Sirius hissed, rubbing his stinging cheek.
“You deserve that and so much more, Black. Do you have any idea what you've done you fucking prick?” Peter yelled at him and pulled his foot back as though he was winding up to kick Sirius right in the rib cage.
Sirius scrambled out of range. He has never seen Peter so angry, or so terrifying, in all the years he’s known him.
“Oh come on, Pete. It’s Snape. We hate that slimy git. He was fucking around with Lily and Regulus. He deserves a bit of a scare, don't you reckon?” Sirius huffed out a nervous laugh. “He’s such a coward, I bet he didn’t even show up anyway and-”
“Oh, well that just makes it so much better. Would you bet Snape’s life on him being too cowardly to show up, huh Sirius?” Peter demanded, his face turning red. “Would you gamble on Remus’ life?”
Sirius looked at him with wide eyes, the realisation of the consequences of his choices only dawning on him now.
“Oh. Fuck, fuck I didn’t think about how it would be for Remus.” Sirius whispered. “I didn’t think about that Pete. I was only thinking about how much I hate Snape, and, and- oh Merlin. I have really fucked up now. Oh fuck, this is bad.” Sirius’ breathing ran away from him. Panic seeped into his bones. He couldn't suck any air into his lungs. His vision blurred around the edges, and he couldn't tell whether he was going to throw up or pass out. Oh fuck, what was wrong with him? How could he do something so cruel to Remus?
“Pull yourself together, Sirius. Breathe.” Peter slapped Sirius across the face again, although more gently this time. “I’m going to help James. You need to go and explain this to McGonagall, Dumbledore, anyone who can help us. Okay?” Peter huffed impatiently. “Fucking go, Sirius. Moony needs us.” That spurred Sirius into action, and he raced out the door towards Dumbledore’s office.
***
“Calm yourself, Mr Black. I can’t assist you if you don’t tell me what the matter is.” Dumbledore said calmly, pouring Sirius a cup of tea. Sirius made a deliberate effort to slow his breathing. “It’s Remus. He’s in trouble, Sir. And Snape, too.”
Dumbledore’s expression faltered, and a shadow passed over his weathered face. “Sirius, I must ask, do you know where Mr Lupin is right now? Because it is my understanding that Mr Lupin is..predisposed tonight.” Dumbledore raised his eyebrows at Sirius slightly, trying to get him to admit just how much he knew about Remus’ condition. Sirius resisted the urge to roll his eyes. They really didn’t have time for this. “We’ve shared a room every night for five years, Professor.” Sirius stared at him incredulously. “Every night except once a month. Yes, I know about his lycanthropy.”
“Right, well.” Dumbledore nodded, “Why do you believe Mr Lupin and Mr Snape are in danger?”
Sirius swallowed down the bile rising in his throat. Tears welled in his eyes, but he would not cry in front of Dumbledore.
“I told Severus how to get into the Shrieking Shack, Sir. I - I told him to go there tonight. I wasn’t thinking. Please, sir. They need your help before something terrible happens. ” Sirius can’t hold back the tears now. They drip down his cheeks, burning more than his parents’ crucio curses ever really did. Dumbledore blinks at him with a shocked, stern expression. And then he apparates away, leaving Sirius shaking and alone in his office. Sirius can’t just wait around. And so, he transforms into Padfoot and bounds out of the castle and towards the Whomping Willow.
***
When Padfoot arrives at the willow, he is overwhelmed by the scents that seem to fill every corner of this part of the world. Padfoot can smell Moony, so he’s been outside the Shack recently, but the aroma is not as prominent as Prong’s earthy scent, now infused with the deep, iron smell of blood. Padfoot tracks Prongs’ scent into the forbidden forest, and stops short. He can hear James sobbing frantically. Padfoot follows the sound, and in a clearing not far from him, he sees James hunched over the limp form of Severus Snape’s bloody corpse. This doesn’t really register in Padfoot’s mind until James is ever so tenderly scooping Severus’ body into his arms, and starts to move slowly towards the castle. It’s well past night fall, and Padfoot’s dark coat blends easily into the shadows. James doesn’t see him. Sirius does not make any attempt to come out of the bushes. It hits him all at once. Severus Snape is dead, and it's all his fault. He killed him. No, worse than that. Sirius sent Severus to his death, and made Moony a killer. Oh, Remus. The weight of Sirius’ shame and self-hatred is soul crushing. How could he ever face Remus again? He had made all of Remus’ worst fears come true, and that was something no one he loved would ever be able to forgive him for.
No matter how hard Sirius had tried to out run his family, it had all been for nothing in the end. Everything he touched would always be tainted Black, leaving permanent stains on everything good in his life. He ruined everyone he came close to.
Sirius had tried to escape his family at every opportunity. Sirius had so desperately wanted to be the good, brave, Gryffindor who escaped the ancient and most noble mania and was undyingly loyal to his friends. He had wanted to be worthy of them and their love. He wasn’t and never would be, and had proven that time and time again. Sirius Black was a selfish coward. He had become the snake Walburga and Orion had always hoped to raise him into. Sirius had nowhere to go and no one left to turn to. So he did what he had always done when things got too hard. He ran away.