Note: I am not asking why people thought Sirius was guilty, and I am not asking why he was convicted. My question assumes as a given that Sirius was guilty of both giving Voldemort the Potter’s location and killing thirteen people in one shot.
There are some claims made about Sirius that seem to have no basis in reality even given that he was guilty of the crimes he was convicted of. For example, in Chapter Ten of Prisoner of Azkaban we find the following:
“You don’t know what you’re talking about, Hagrid,” said Fudge sharply. “Nobody but trained Hit Wizards from the Magical Law Enforcement Squad would have stood a chance against Black once he was cornered.
This does not seem to be congruous with what we know. First of all, we have seen Hagrid take on multiple Death Eaters at once without any major negative consequences. Second of all, while Sirius was probably a skilled wizard, we don’t find any indication that he was extraordinarily powerful such that only trained hit wizards would be able to deal with him. He loses in a duel to Bellatrix, whom a trio of teenagers who hadn’t completed their magical education were able to handle. In general, Death Eaters seem to have been able to be handled by ordinary members of the Order of the Phoenix and Dumbledore’s Army, none of whom were trained hit wizards.
And it’s not like people didn’t know Sirius. Until the day before his “crimes” he was a regular member of the Order of the Phoenix. People knew him from school, people fought alongside him, people were familiar with his magical prowess. So how did he suddenly the next day become this supremely powerful wizard that no one but trained hit wizards would be able to handle?
Another quote from the same chapter:
“I daresay that is his—er—eventual plan,” said Fudge evasively. “But we hope to catch Black long before that. I must say, You-Know-Who alone and friendless is one thing . . . but give him back his most devoted servant, and I shudder to think how quickly he’ll rise again. . . .”
The description of Sirius as Voldemort’s devoted servant seems highly exaggerated as well. Voldemort had a number of servants who are at times implied to be his closest followers - e.g. Bellatrix, Barty Crouch Jr., Snape, Lucius Malfoy. There is nothing to indicate that Sirius had any unique devotion. Until the day before he was thought to be a regular member of the Order of the Phoenix. The only thing that changed since then was that he betrayed the Potters and killed a bunch of people. Neither of those indicate anything more than that he was a dark wizard who supported Voldemort.
Similarly, in Chapter Three we find:
“So—so Black was a supporter of You-Know-Who?” Harry prompted apologetically.
“Yeah,” said Stan, still rubbing his chest. “Yeah, that’s right. Very close to You-Know-’ Oo, they say. Anyway, when little ’Arry Potter got the better of You-Know-’ Oo—”
Harry nervously flattened his bangs down again.
“—all You-Know-’ Oo’s supporters was tracked down, wasn’t they, Ern? Most of ’em knew it was all over, wiv You-Know-’ Oo gone, and they came quiet. But not Sirius Black. I ’eard he thought ’e’d be second-in-command once You-Know-’ Oo ’ad taken over.
Again, there does not seem to be any evidence that Sirius was closer to Voldemort than the average Death Eater, and certainly not that he was seeking the second-in-command position.
How did these ideas about Sirius proliferate when nothing seems to indicate that he was more than a regular guy who became bad?