Sirius was hiding in a cave in Malta when he got the letter. It was Harry's owl. The one he had got him whilst he was still in England. The sight of it made him a little bit angry, he had told him not to write, it was too risky, all owl mail could be traced with a single, very simple spell. Still, he opened the letter, and started to read. Immediately tears sprung to his eyes, he hadn't expected this, not one bit. This was his worst nightmare coming true. Harry wanted to die, or so he said. At that thought a small flicker of hope came into Sirius' eyes. Maybe he wasn't going to do it, maybe it was a trick, a hoax to bring him back home. Still, he had to check, had to make sure harry was all right. He couldn't let Lily and James' son kill himself.
He gathered his energy for the apparition, long distances were always tricky for him, but he knew he had to get there as soon as possible. With a loud crack, he disappeared. He reappeared in Surrey, Little Whinging to be more specific, right outside number 4. Privet drive. One simple Alohamora and he was inside. What he saw shocked him, tore his heart apart. There was Harry. Lying dead on the floor, in a puddle of his own blood, and a knife in his heart. Not even in his twelve years in Azkaban were worse than this image. 12 years of torture and dementors, and it had taken less than a minute to truly break him. And that's what he was: broken. His heart shuddered in his chest, and stopped, he stopped breathing. Tears poured down his face, he had let them down, Lily, and James. Oh god, James he thought. He was horror struck, was so disappointed in himself, that he had let them down, in such a horrific way.
He fell to his knees, uncaring of the blood soaking into his threadbare trousers, the blood that was slowly coating his hands as he ran his fingers through Harry's hair. This wasn't how this was supposed to go. He was supposed to get free, move back to Grimmauld Place, with harry. They were supposed to be a family. Then he saw. He saw the cuts going up and down Harry's arms. He knew those scars. How couldn't he? He had his own to match. His already twisted heart shattered at the sight, that someone so young had seen so much, been through so much, that this is what it would come down to. But still, he had to see more, he knew arms weren't the only place someone could cut themselves, not by a long shot. Then he saw the scars on his back. Long, and thick, and angry. Some of those were no more than a week old, some were years old. He knew than that harry hadn't been exaggerating, he was truly tortured in his own home. And Sirius had done nothing. IT tore through him like a knife, like lots of knives, hundreds of the, tearing at his skin, for failing Harry so badly.
Then came the rage. He was angry. How could Harry do this, what was worth taking his life? All Sirius saw was red, red like the blood surrounding Harry's body. His fingers started twitching, his hands started repeatedly clenching into fists, and then uncurling, just so clench once more. He threw things, pots, pans, vases. When the muggles came downstairs to see what all the racket was, he was murderous. It was only the thought of disappointing Harry, and his parents, bless their souls, that stopped him from killing them where they stood. H ran. Out of the destroyed front door, and into the night. Running from his nightmares, from this living nightmare, from everything that house represented: his past, the abuse he suffered from his father; the present, the truth that he couldn't even begin to comprehend, that he was so actively trying to avoid, because he knew, if he stopped, it would bring him to his knees. And the future. Sirius knew about the prophecy, the one Dumbledore was trying so hard to hide from the world. He knew that without harry, Voldemort could never truly die. And that thought scared him. It would scare anyone, anyone that knew what life could be like if Voldemort won. It was that though, that and his memories of the 1st war, that caused him to keep running, further into the night, into the darkness, where he knew no one would, could, find him. And he was right. No one ever saw him again. He was a memory, a shadow, a whisper.