In Philosopher's Stone, Hagrid says that "there's not a single witch or wizard who went bad who wasn't in Slytherin," and yet Sirius Black, a Gryffindor, was still considered to be one of the most dangerous and evil wizards alive at that time. How did the community at large explain Sirius's being a Gryffindor while he was still believed to be a faithful follower of Voldemort? Might they think the Sorting Hat makes mistakes?
Was the community surprised? I'm sure they were surprised that a dear friend of the Potters was accused of betraying them.
However, to the second part, it is wrong to assume that all Slytherin are evil and that the only evil wizards get sorted into Slytherin (or vise versa).
Some examples of evil and good among houses:
Wormtail is a Gryffindor who turned Death Eater.
Quirinus Quirrell might be considered a Death Eater, from Ravenclaw.
Although not a Death Eater, Lockhart was certainly an evil wizard, even though he came from Ravenclaw.
Slughorn was from Slytherin, and while he had great ambition, he wasn't evil or a Death Eater.
Hufflepuff House is said to have boasted of being the only house never to produce any Dark Wizards.
The wizarding community at large seems more interested in the idea that Sirius betrayed his best friend rather than how his betrayal could be explained in terms of his Hogwarts House. While wild rumors do fly around, claiming that Sirius was close to Voldemort, people with more information seem to view him primarily as a traitor, rather than as a Dark wizard himself. Hagrid, for instance, tells Harry that all Dark wizards have come from Slytherin, despite the fact that he was well acquainted with Sirius.
The most likely reason that the connection between Sirius, Dark wizardry, and Gryffindor was never made is Sirius's family background. His name is Sirius Black. He comes from a family of pureblood supremacists. His brother Regulus, his cousin Bellatrix, and his cousin-in-law Lucius Malfoy were all Death Eaters. People may have viewed him as a surrogate Slytherin, despite his actual Hogwarts House. Sirius fought against that stereotype all of his life, but people who did not know him personally might not be aware that, among other things, he fought with his parents for years until they finally kicked him out of the house.
A contributing factor might be Sirius's behavior after he was arrested. Sirius seems to have been so emotionally overwrought by James Potter's death and Peter's double betrayal that he suffered a nervous breakdown. Laughing in the street, after Peter had just framed him? Sirius, quite frankly, lost control. And with that control went any chance of convincing anyone of his innocence. Other people may have concluded that the Blacks were a little unstable anyway, and Sirius had obviously gone over the edge.
Merlin was in Slytherin and considered one of the greatest wizards of all time.
Slughorn was the head of house for Slytherin and ended up fighting on the side of the Order.
Simply being in Gryffindor did not preclude someone from committing crimes or being a generally bad person.
Not to mention, Sirius did not even contest the charges at the time. He felt he no longer had anything to live for, so he went quietly with the Ministry of Magic law enforcement laughing his head off insanely.
How did the community at large explain Sirius's being a Gryffindor while he was still believed to be a faithful follower of Voldemort?
Note: By the community I understand wizards who have finished wizarding education and have some occupation.
I don't recall the community ever take into consideration Hogwarts houses. Do they give ex-griffindors Auror positions more willingly? Are the ex-slytherins tried in court more thoroughly? The deeds is what matters, not the School house. Or the School itself as a matter of fact. Did anybody pay attention to Karkarov's school when he was caught? God no! Therefore, there was no need to explain anything. The more horrifying thing to explain at that moment was that he was their friend and Harry's godfather rather that a schoolhouse he belonged to several years prior.
To put it simply, wizards stop being griffindors, slytherins, hufflepufs and ravenclaws once finished school. They become ministry officials, potion makers, journalists, aurors... you get the idea.
The only thing some of them do pay attention to though is the pureness of the blood, but that's another story.
Might they think the sorting hat makes mistakes?
Here we must take into consideration my previous thoughts that nobody actually cares about the wizard's house or his former school at all, it does not influence their future adult life. Will that mistake seem truly big of a deal? In fact... do adult wizards ever think about the Sorting Hat at all? )) It was made to put their kids into a particular part of the castle for 7 years and that's it. All of them get the same teachers, same subjects, same knowledge... so forth. House names don't matter for adults. You see an example in the final book (especially for @DVK who seems to be asking everyone for cannon stuff :p ))) ) that Harry Potter told his son that the Slytherin House would get an excellent student if the Sorting Hat put him there. Was Harry some exception? I guess no... he was adult.
The evidence made him seem guilty - his house didn’t matter.
Though we know he was innocent, Sirius Black looked quite guilty at the time. He’s described as laughing at the scene of the crime where he was found, in the middle of chaos where Muggles were just killed and Pettigrew ‘dead’ with his finger as left behind as evidence. Fudge, who was one of the Ministry officials handling the scene, described it, and Sirius seemed quite unhinged.
“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 blood-stained robes and a few – a few fragments –”
- Harry Potter and the Prisoner of Azkaban, Chapter 10 (The Marauder’s Map)
This is the story that the wizarding world knows about Sirius Black - he killed thirteen people with one curse, then laughed about it. He was both present at the crime scene and seemed pleased. Without actually knowing the full story, that sounds like pretty solid evidence indicating his guilt.
“Anyway, they cornered Black in the middle of a street full of Muggles an’ Black took out ’is wand and ’e blasted ’alf the street apart, an’ a wizard got it, an’ so did a dozen Muggles what got in the way. ’Orrible, eh? An’ you know what Black did then?’ Stan continued in a dramatic whisper.
‘What?’ said Harry.
‘Laughed,’ said Stan. ‘Jus’ stood there an’ laughed. An’ when reinforcements from the Ministry of Magic got there, ’e went wiv ’em quiet as anyfink, still laughing ’is ’ead off. ’Cos ’e’s mad, inee, Ern? Inee mad?”
- Harry Potter and the Prisoner of Azkaban, Chapter 3 (The Knight Bus)
The evidence was against him. After people heard that, his Hogwarts house wouldn’t matter. Being a Gryffindor wouldn’t be enough to cast doubt on his guilt, even for people like Hagrid, who were biased in favor of Gryffindor - not with what seemed to have happened. Hagrid (who’d said all bad wizards were in Slytherin) thought he was a ‘murdering traitor’. Even people who knew not just his house but had known Sirius personally and wouldn’t have thought him capable to do something like that, had been convinced he did it. Though Madam Rosmerta had trouble believing it, she still did.
“Do you know, I still have trouble believing it,’ said Madam Rosmerta thoughtfully. ‘Of all the people to go over to the Dark side, Sirius Black was the last I’d have thought … I mean, I remember him when he was a boy at Hogwarts. If you’d told me then what he was going to become, I’d have said you’d had too much mead.”
- Harry Potter and the Prisoner of Azkaban, Chapter 10 (The Marauder’s Map)
In addition, being a Gryffindor isn’t proof of goodness, just bravery (or the valuing of it). A person who was seemingly guilty couldn’t be proven innocent by their Hogwarts house - all the houses have had some number of ‘bad’ wizards. People probably didn’t even think about what his house was after hearing of how Sirius Black had killed thirteen people with one curse, then laughed. At that point, house generalizations would have seemed irrelevant in the face of the evidence.