Hagrid once said:
"There's not a single witch or wizard who went bad who wasn't in Slytherin."
Which simply isn't true. Take these awful people as examples:
So why did Hagrid say all bad people came from Slytherin?
I think perhaps Hagrid was simply prone to exaggeration.
Sure, a significant proportion of evil people in recent history were from Slytherin (i.e. the Death Eaters), but as you have pointed out this does not mean everyone was. At this point in time everyone thought Sirius was Voldemort's biggest supporter, and he was a Gryffindor.
However, this is a statement coming from a person who constantly tells people that Dumbledore is "The Greatest Wizard Who Ever Lived". Whilst this may be true, no one else refers to him this way. Sure, they think he's pretty great, but no one is quite so aggrandizing as Hagrid.
I'm sure there are more examples of Hagrid overstating things, but I don't have the relevant sources at hand to choose quotes from.
Out-of-universe, J.K. probably just wanted to use this to establish that the Slytherin house tends to attract the type of people who would be more prone to becoming dark witches and wizards, hence setting up the rivalry of Harry with Malfoy, and casting more suspicion on Snape being the person supporting Voldemort and trying to get the Philosopher's (Sorcerer's) Stone as the head of the 'evil house'.
When Hagrid said “there wasn’t a single witch or wizard who went bad who wasn’t in Slytherin”, he wasn’t likely intending precision or complete accuracy. Hagrid probably knew of a lot of Dark wizards from Slytherin, so told Harry they’re all from Slytherin without stopping to check the accuracy of what he was saying.
“School houses. There’s four. Everyone says Hufflepuff are a lot o’ duffers, but –’
‘I bet I’m in Hufflepuff,’ said Harry gloomily.
‘Better Hufflepuff than Slytherin,’ said Hagrid darkly. ‘There’s not a single witch or wizard who went bad who wasn’t in Slytherin. You-Know-Who was one.’
‘Vol– sorry – You-Know-Who was at Hogwarts?’
‘Years an’ years ago,’ said Hagrid.”
- Harry Potter and the Philosopher's Stone, Chapter 5 (Diagon Alley)
Quite a few Dark wizards were in Slytherin, so it’s not entirely unfounded - it just doesn’t hold up to close scrutiny, since there are counter-examples. The way he put it, it can be disproved by only one example of a Dark wizard from another house. People with analytical minds tend to avoid making absolute statements for exactly this reason. However, Hagrid says what he thinks but doesn’t always think about what he says. It’s likely that Hagrid simply couldn’t think of any Dark wizards from another house, and associated Slytherin with Dark wizards, so made his statement to Harry based on that.
In addition, he’s not really the type to mentally double-check what he says, it’s unlikely he was listing all the Dark wizards he knew in his mind to make sure none were in another house. He was trying to explain Hogwarts houses to Harry, who knew absolutely nothing about them - since Hagrid didn’t think Slytherin was a good house to be in, that would come across when he’s trying to give Harry descriptions of the houses and what they represent. Hagrid himself knew of one counter-example (though it turned out to be false) since he thought he knew Sirius was the one who betrayed the Potters. He definitely knew this, and hated Sirius for his supposed betrayal. He also knew Sirius was a Gryffindor, so he would know the person who (supposedly) betrayed the Potters and killed Muggles and Pettigrew wasn’t a Slytherin.
“Jus’ got him outta the ruins, poor little thing, with a great slash across his forehead, an’ his parents dead … an’ Sirius Black turns up, on that flyin’ motorbike he used ter ride. Never occurred ter me what he was doin’ there. I didn’ know he’d bin Lily an’ James’s Secret Keeper. Thought he’d jus’ heard the news o’ You-Know-Who’s attack an’ come ter see what he could do. White an’ shakin’, he was. An’ yeh know what I did? I COMFORTED THE MURDERIN’ TRAITOR!’ Hagrid roared.”
- Harry Potter and the Prisoner of Azkaban, Chapter 10 (The Marauder’s Map)
Hagrid does have a tendency to make broad generalizations, without considering the precise accuracy of his statements. He also says no Malfoy is worth listening to - it’s impossible to think that he could have met every single Malfoy and know for a fact that they’re all not worth listening to. There may be a Malfoy who is to that family what Sirius is to the Black family, a pariah who rejects the ideals the rest of the family believes in, but Hagrid is most likely simply referring to the Malfoys he knows and thinks of first upon hearing the name.
“Yeh should’ve ignored him, Arthur,’ said Hagrid, almost lifting Mr Weasley off his feet as he straightened his robes. ‘Rotten ter the core, the whole family, everyone knows that. No Malfoy’s worth listenin’ ter. Bad blood, that’s what it is. Come on now – let’s get outta here.”
- Harry Potter and the Chamber of Secrets, Chapter 4 (At Flourish and Blotts)
This is another good example of something that’s probably mostly true (at least from the perspective of the protagonists), but as an absolute statement, can be disproved by one Malfoy who doesn’t share pure-blood ideals.
Well, as a house, Slytherin did seem to attract all the 'evil types'. All the Death Eaters (apart from Pettigrew) are from Slytherin. Not to mention old Voldy himself.
Also, within Hogwarts, all the bullies were from Slytherin. The Slytherin team did not play fair in Quidditch. All the nasty incidents of student clashes involved Slytherins.
Then there is Salazar himself. Yes, he was a founder of Hogwarts and one of the greatest wizards ever. But he did create a 'Chamber of Secrets', which held a Basilisk, one of the most dangerous magical creatures ever. And he put this Chamber in a school full of young witches and wizards. If that's not evil, I don't know what is. With the founder of Slytherin being evil, its no surprise that the students sorted into it turn out that way as well.
Of course, it was a house full of adolescents. So maybe many Slytherin students were peer-pressured into being evil, and turned out to be normal magical citizens in the end.
Also, it seems Quirrel was off doing research when Voldemort possessed his body. We don't know if Voldemort forced himself upon Quirrel or he voluntarily accepted being possessed. As for Pettigrew, it is said he was threatened into becoming a Death Eater, so Voldemort could get close to the Potters. Of course, he was all in after that. But I doubt he wanted to become Death Eater of his own volition.
There is of course Barty Crouch Jr. It doesn't say which house he was from in school, but we can be fairly certain, considering who his dad was, that if he had been in Slytherin his dad would have disowned him right then and there. So he might be an exception to the 'all bad people are from Slytherin' rule.
So no all Slytherins are not evil. And all evil people are not from Slytherin. But most evil people are from Slytherin and that is what Hagrid was trying to portray I think. He said it like that because he wasn't trying to be politically correct. Also, he really did not want Harry to think Slytherin was an option as a house. Later in the first book, we see that the Sorting Hat put Harry in Gryffindor only because he kept thinking "not Slytherin" in his head. This was probably the seed of that thought.
Hagrid must either be exaggerating or in error. For one thing, we know that there are other wizarding schools aside from Hogwarts, and we know that not all witches and wizards who turn bad are from the UK. Unless Hagrid is suggesting that only English wizards are evil, we can't take what he says at face value.
I think people often take this a little too literally. I believe Hagrid meant that as a broad assumption. To rephrase what Hagrid originally said (and my interpretation of it): "if you do come across a bad witch or wizard, it's more than likely they came from Slytherin."
When you look at it, Slytherin has certain traits that the sorting hat identifies, in order to place them in that house:
When looking at this list, there are certain traits that you can definitely see as characteristics of an "evil" person; particularly cunning, ambition, and self preservation.
However, that does not mean that all Slytherins are bad people. Some notable names of the Slytherin house:
And then some others while they do carry out some nasty things, they are not entirely done of their own desire; particularly in the case of Draco Malfoy; who was more forced into his evil role, rather than choosing it. It was simply his characteristics of self-preservation that kept him from doing it of his own free will.
And applying this logic inversely, there are other wizards and witches from other houses that have turned evil. Looking at the traits of the Gryffindor house for example:
Pettigrew is a prime example of how these traits can still exist in an evil person. He was the only one brave enough to return to Voldemort; his nerve when it comes to killing others (simply a flick of the wrist) as a couple off the top of my head.
All that being said - the volume of "evil" witches and wizards do primarily come from Slytherin, and the sheer volume of them that joined Voldemort's ranks is a show of that.
Hagrid has had a very difficult life, and the evil wizards in Slytherin have harmed him personally. For one, the Dark Lord comes from Slytherin, and has killed many of his friends, just like for most wizards in the war.
More importantly, when he was a young student in Hogwarts, Headmaster Dippet believed that he released Slytherin's monster and killed his fellow student Moaning Myrtle with it. As a result, Hagrid was expelled from Hogwarts, his wand was broken in two, and he got denied any hope that he'll be integrated into the community of wizards as a normal person. Hagrid knew that he didn't open the Chamber of secrets, and that whoever killed Moaning Myrtle wasn't one of his monsters. It was Slytherin's Heir. As such, Slytherin's Heir, whoever they were, have personally destroyed Hagrid's chance for a good life. He's had to deal with shards of a broken wand in his umbrella, and it was only with Dumbledore's help that he even got any job at all.
