A/N: This is a one-shot that relates to my story 'The First Wizarding War'. I cannot comprehend how Dumbledore doesn't see Sirius was innocent in 1981. Here is my attempt to answer my own question!
...
Dumbledore sank into the armchair and put his head in his hands. He'd done it again. He'd trusted again. What a silly. Old. Fool.
He felt angry. It wasn't an emotion he was at all used to feeling, as he so rarely had need of it. Most people he understood. He saw through his students' attempts to pull the wool over his eyes, their funny excuses, their clever attempts to avoid facing responsibility.
And Sirius Black had been one of them.
Dumbledore remembered meeting him for the first time at Hogwarts. He'd been on his way to the kitchens for hot chocolate, pondering vaguely about the room full of chamberpots he'd recently discovered and whether it was another of the Prewett twins' practical jokes (and also wondering how he could convince Minerva McGonagall he took the crime seriously enough for her not to suspect him of siding with the students).
He'd paused on his way downstairs as he heard the unmistakable sound of weeping from a side passage.
Albus had never shied away from the suffering of others. While he'd seen first hand students awkwardly go back another way when stumbling across a crying first year, the sound of the tears had quite the opposite effect on him.
"Mr Black?" He said gently, recognising the small boy from his sorting as he approached him.
Sirius, who'd been huddled on a stone ledge with his head in his hands, jumped and looked up. He hastily wiped his eyes and put his feet quickly back on the floor. "Er, sorry sir," he said, fixing Dumbledore with an artificial smile. Dumbledore thought he saw him stuff a letter in the pocket of his robes.
"You've nothing to apologise for." Dumbledore said. Eleven year olds cried all the time. Every week Madam Pomfrey had one in the hospital wing needing a calming draught for homesickness.
Sirius looked as though he didn't believe him. He checked his watch and Dumbledore saw his face pale slightly. "Er…"
Dumbledore smiled. "It's alright. I don't mind you breaking curfew every once in a while." Oh wait a minute, hadn't Minerva told him Sirius Black and James Potter had been caught breaking curfew on their second night of term? He really needed to stop being so relaxed…
"Besides, I can understand you'd want to be alone when you feel like this." Dumbledore said, smiling at him again. "But I also know that telling someone what's going on can help a lot."
He wondered if Sirius would take him up on the offer to talk. Some students did. Many others didn't. He thought he could guess what side Sirius would fall on.
"Thanks, Professor." He said politely, getting to his feet. "I'll go right up."
Dumbledore watched him go with that odd sense of powerlessness and sadness that he sometimes felt when he observed children like Sirius.
He knew the Black family well. He remembered Orion and Walburga Black from his days as a transfiguration teacher and of course he'd encountered Sirius' grandparents over the years too.
He had frequent debates with Phineas Nigellus Black (Sirius' great-great-grandfather) about how to be a good and fair headmaster, none of which he would care to share with anyone, as the man was, in Dumbledore's opinion, far too odious a creature to give any airtime to.
He did feel a rush of empathy for their relatives though. Treating children cruelly surely wasn't limited to Phineas Nigellus' time in the castle, and given the Blacks' reputation and from what he'd seen of Sirius' behaviour, he was sure poor Sirius had had a rough time of it already.
But that was why Dumbledore loved being a headmaster. He could show his students another way. Whatever background they came from, he would help them to thrive at Hogwarts. It filled him with the most spectacular joy to see his students come into themselves, and never had he felt this as much as with Sirius Black.
He'd known Sirius was different and that he was strong from the moment the hat had declared him a Gryffindor. Of course they all kept up a big pretence that the sorting hat made the decisions, but everyone knew it was really the individual in the end.
He'd been quite unable to stop himself smiling as young Sirius made his way over to the sea of red, and had followed the rest of his school career with interest.
Again displaying his woefully unprofessional attitude to practical jokes, he had tried not to laugh as Professor McGonagall continued to report Sirius to him. If he was being honest with himself, Albus admired the boy's spirit. It can't have been easy facing the wrath of his parents in the holidays, and it pleased him to know he could exercise some freedom and creativity in the castle, even if many of his fellow educators were shocked by it.
He was glad to have offered Sirius his freedom. In his second year he'd successfully helped two undercover aurors bring Corban Yaxley in for questioning at the ministry (even if he had broken a few school rules in the process).
Dumbledore remembered the chat he'd had with Sirius in his office afterwards. He'd wanted him to see it was OK to be vulnerable. It was OK to talk. And it was OK to trust.
He'd hoped the incident with Snape and the willow in his fifth year might have helped him to see that. Despite being shocked that a sixteen year old could act quite so stupidly, he'd had compassion for Sirius. He could see just how close he'd come to following in his parents' footsteps and (in his opinion at the time) had guided him to see there was another way.
He remembered meeting him in the grounds the morning after the incident. He clearly hadn't wanted to face James or Peter in Gryffindor tower and had chosen to sleep outside, by the willow, instead.
He'd been a good person. And he'd cared for his friends, just as much as they'd cared for him.
And they had cared for him. James Potter had taken him in just months after the willow incident. Dumbledore remembered the conversation he'd had with Sirius in his office, after Minerva had insisted that he go and tell him.
He'd given Sirius an allowance for school uniform and equipment (it seemed he'd left in too much of a hurry to pack properly and his parents had not been in touch), and, of course, asked the boy how he was feeling.
Sirius had smiled at him. A confident, genuine, self-assured smile. Not the blustering grin of fools who tried to convince the world they were someone special while battling crippling inadequacies, but the warm, steady smile of someone who knew they'd done the right thing.
"Never better, sir." He'd said, and of course Dumbledore had believed him.
And so this was why he couldn't understand it. This was why he was so angry.
Sirius Black had thrown away his chance to be a member of the Black family at the age of sixteen. Dumbledore knew that would have been an easy way in for him with Voldemort, if that was what he wanted. Sirius' cousins Bellatrix and Narcissa were both married to death eaters, after all, and his brother had been killed a couple of years ago while on duty too.
Why had Sirius decided at the age of twenty one that he was going to go back on ten years' friendship and rethink his decision at sixteen?
He had hated his family. Dumbledore remembered having both him and Regulus Black in his office after another fight in the corridors.
"Things in common?!" Sirius had said incredulously as Albus made the suggestion for the exercise. "The only thing I've got in common with that cretinous bastard is my last name!"
The animosity went both ways. "I'm glad you moved out!" The younger boy had shouted. "Now finally we've all got a chance at happiness."
"Happiness?!" Sirius laughed incredulously. "Twits like you and our parents aren't ever happy. You'll just make your misery someone else's fault so you never have to face it. Well I'm sick of being your fucking scapegoat!"
Dumbledore hadn't bothered to correct his language. He thought Sirius just about had the measure of the Blacks. The fact that he could see through them was admirable, the fact that it hadn't driven him mad, even more so.
But he was mad. He had to be mad. How else could he have betrayed Lily and James Potter?
Dumbledore buried his head in his hands again. What a stupid, foolish idiot he was. How could he have believed someone could change as Sirius had? How could he have believed someone might be able to escape the system from which they were born?
The rage he felt was quite unlike him. He wanted to smash the whisky bottle on his side table. He wanted to bellow at Phineas Nigellus Black's portrait that he was right. And was he happy now?!
How could Sirius Black be going to Azkaban? He'd been so happy, so well-liked, so cared for. He'd had friends. He'd been a godfather. His life was going so well.
He'd been one of Dumbledore's success stories. Of course Dumbledore knew he shouldn't really have 'success stories' amongst his students, and that as headmaster he needed to be fair with everyone, but he couldn't help it. It filled him with so much joy to see children rebelling from their parents, particularly if their parents were Orion and Walburga Black.
Most children tended to do what they were told, generally. Bellatrix and Narcissa both had, as had Regulus. It was rare to see the level of individual spirit he'd seen in Sirius, and it was seeing that courage, determination and resilience in people that made his job so worthwhile at the end of the day.
But not this time. He'd been wrong this time. He must have got Sirius wrong. What a fool. He was always doing that. Trusting people. Believing the best in them.
No one else did. The world was full of cynics and maybe that was just the way it went. There's no such thing as an altruistic act. There's no such thing as true kindness. People always want something. You can't trust anyone.
Oh how many times had he been told that growing up? How many times had he been called naive? A bleeding heart, a do-gooder, a dreamer? Well, they were all right. He hoped they were celebrating. He'd got it wrong.
The world was a terrible place. Because Sirius Black had betrayed James Potter.
...
A/N: Well, it DOES have a happy ending as, thank Merlin, Sirius didn't betray James! If anyone's interested in my interpretation of how things actually went, please do check out my other story.