Why are claims about Sirius so exaggerated?

Question

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?

Answer 1

You state:

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.

Yes, people were familiar with his abilities... and he'd proven at school to be quite adept at magic. Even though people didn't know that he was an Animagus, he was at least at the level of James and Remus - which certainly wasn't low. People knew he was a very talented wizard - McGonagall mentions something about it at the Three Broomsticks.

"Precisely," said Professor McGonagall. "Black and Potter. Ringleaders of their little gang. Both very bright, of course — exceptionally bright, in fact — but I don’t think we’ve ever had such a pair of troublemakers —"
_{Harry Potter and the Prisoner of Azkaban, chapter 10: "The Marauder's Map}

Pettigrew framed Sirius for killing an entire street:

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. I was Junior Minister in the Department of Magical Catastrophes at the time, and I was one of the first on the scene after Black murdered all those people. I — I will never forget it. I still dream about it sometimes. A crater in the middle of the street, so deep it had cracked the sewer below. Bodies everywhere. Muggles screaming. And Black standing there laughing, with what was left of Pettigrew in front of him... a heap of bloodstained robes and a few — a few fragments —"
Fudge’s voice stopped abruptly. There was the sound of five noses being blown.
_ibid

"Just before he transformed," said Black. "When I cornered him, he yelled for the whole street to hear that I’d betrayed Lily and James. Then, before I could curse him, he blew apart the street with the wand behind his back, killed everyone within twenty feet of himself — and sped down into the sewer with the other rats..."
_{Harry Potter and the Prisoner of Azkaban, chapter 19: "The Servant of Lord Voldemort"}

So people knew that he was an exceptionally talented wizard, who had just sold out his best friend and killed an entire street in cold blood. Plus apparently totally insane.

As far as being second in command to Voldy, that's simply the kind of rumor that would spring up around such a powerful wizard who seems to have simply snapped once Voldy was defeated. And we know just how easily the Wizarding World will believe something (coughRitaSkeetercough) - once the rumor's been started for whatever reason, without anybody refuting it people are going to believe it.

Also, we don't quite know just how much of a reputation he had before he broke out of Azkaban. One of Voldy's followers escaping from Azkaban is highly likely to get people talking and new rumors spreading - it's not inconceivable that that particular myth only sprung up afterwards.

Answer 2

This is just my opinion since there isn't many info about this.

How did these ideas about Sirius proliferate when nothing seems to indicate that he was more than a regular guy who became bad?

-- Before escape from Azkaban: Sirius fight with Peter in the timeline where Voldemort just dead. Ofc news about capturing powerful Death Eater will calm the society's public. So propaganda about Sirius's ability may be exagerrated.

-- After escape from Azkaban: He is the first known person to escape from Azkaban prison (if you count Barty Crouch Jr). Something nobody had ever done. Ofc this will a big news in the time where Voldemort is assumed to be death.

-- Possible reason: This is a magical world. If you don't exagerrate like terrorist is coldblood and they always kills ppl without emotion, ppl will try to hunt them and become famous. Take an example of Corona epidemic, will you tell the truth to people or exagerate some fact so people take precaution measure like washing hand

Answer 3

Like any political strategy he was simply a pawn in the ministry of magics chessboard a supposed threat to thier law and order and a easy target for them to blame who knows how many minstry officials used to be voldemort supporters before they became succesful and the ministry thought sirius knew too many secrets and had to be put in his place

Take Umbridge for example a master manipulator

JK Rowling wanted a diverse antagonist full of good and bad on both sides to make a compelling story

Thats your answer