This story was originally a one shot, but upon rereading it, I really wanted to tackle what Sirius must have felt upon that moment of realization. Mentions of slash, but nothing graphic.

What a fool he'd been.

How? How could he have made such a colossal mistake?

One of the cleverest boys in school. That's what everyone had always said. And yet, when it counted – and not just this time, but any time it truly counted – he always became an idiot. A dangerous quality.

Remus.

How could he have ever—

Stumbling out of the rubble of the Potter's house, away from the unmoving body of his best friend – no his brother – Sirius looked around wildly, as if Remus would be there waiting for him, shaking his head in disappointment.

There was no one. Hagrid had flown off with his motorbike and Harry and the rest of the street was silent.

How could he have ever—

He apparated back to the flat. He needed to see Remus. James and Lily were dead, and it was his own fault. He'd handed them over without even realizing it. All because he was too much of an idiot to trust the only person in the world that he should have trusted.

"Let me do it."

That's what Remus had said. Sirius stomach lurched and he tripped his way to the trash bin, where he proceeded to retch up all the food he'd eaten that day. He'd been so sure. So sure that those words meant that Remus was the enemy. That he was trying to get the information to give to Voldemort.

Instead, Remus had been trying to protect him. To keep him from harm's way, and protect James, Lily, and Harry at the same time.

How could he have ever—

"Remus!" he called, once his stomach was empty. "Moony!"

Silence greeted him and at once Sirius remembered why. He had told him, only a few hours ago, that their relationship wasn't working, and he expected Remus to clear out. Then he'd left. Jumped onto his bike and gone to see Peter, because Remus had looked very much like he was ready to cry, and Sirius had felt sick. He'd needed to see a friend, and James… well James was in hiding, and not even Sirius could get to him.

But Peter wasn't there. And slowly, far too slowly, Sirius had realized why.

Now, for the second time that evening, an icy dread seeped through him.

He hastily stumbled into each room of the flat, looking for Remus. Bookshelves were empty in the living room, the tea kettle – Remus's tea kettle – was no longer on the stove. He threw open the wardrobe, only to find that half of it was empty.

Remus had cleared out. Just as Sirius had told him to.

How could he have ever—

Where? Where would he even have gone? What kind of monster was Sirius? Remus worked at the library. He was barely scraping together enough money to manage sharing expenses with him. What was Sirius thinking, throwing him out?

He looked around, suddenly helpless, and his eyes fell on the framed picture sitting on the nightstand. It was Remus's, but it had always been one of Sirius's favorites. Remus had left it for him. He picked it up with trembling hands.

It was the four of them, back at school. Sirius had his arm slung around Remus's shoulders, and the tawny haired boy was giving him that quiet smile that Sirius loved so much. James suddenly tackled Sirius and the two of them began to wrestle, Remus rolled his eyes and stepped over them both, and Peter was standing to the side, laughing nervously.

Sirius's eyes fixed on the small round boy.

Peter.

He set the picture down, straightening as red hot rage began to fill him. Peter had been the spy this whole time. Peter, who had sat beside him sympathetically while Sirius had told him, James and Lily his suspicions of Remus.

How could he have ever thought that Remus would betray them?

Sirius had fucked everything up with his stupidity. Now he'd fix it.

How could he have ever trusted that rat?