Author's Note: So, from the request of can-o-soup, who correctly guessed the last two characters by the way, kudos, I am adding Dave Karofsky to the character list. In the beginning, I don't think he'll have a lot of interactions with most of the characters, except for perhaps Puck, Kurt, and Blaine, and I think he'll have less storylines in the sense that they won't connect with a lot of the other characters in the beginning.
And while I do think that you're right about the reasons behind why David bullies, he's still going to be in Slytherin. Let me explain.
It was mentioned that his friendship group led him to bully Kurt, and while that may be true, he still did bully Kurt. I'd like to make the brief comparison to Snape for example, who called Lily a mudblood despite being her friend, and was in Slytherin even though Dumbledore mentions later that perhaps Hogwarts sorts too early.
So yes, I do believe Dave is not honestly a bad person, and that he doesn't deserve to be in Slytherin, but that's where he'll be. His 'group of friends' will be the other Slytherins that have a bad habit of bullying both Blaine and Kurt, and so Dave is really only following in their footsteps.
As a last note, this will not be as much of an origin story as many of the others, because it'll start in Dave's time at Hogwarts, and explain more of the reasons behind his bullying and how it escalated into him turning into someone he's not.
So I'm sorry I'm going against the second half of the request to not put him in Slytherin, but I hope my explanation can provide you some comfort with my reasons.
That was pretty much the longest author's note ever, so I'll just skip to the part where I actually do some writing.
The hat sat on his head for a really long time. It was muttering something about "a worried mind" and a "troubling decision" so Dave just sat there quietly. He was surprised when the hat asked him a question directly. "So, where to put you?" the hat whispered into his ear, and Dave didn't really know what to say. His entire family had been in Slytherin, and he would rather die before he'd admit it, but he didn't care where he went. His family had always preached about the whole pureblood thing for as long as he could remember, it just didn't affect him. He never thought he was better than anyone else, he was just always treated that way.
So he didn't care. Even as the hat was pulled off of his head after screaming Slytherin! to raucous applause from the table on the far end of the room, Dave was indifferent. He wasn't better than anyone else, he was just indifferent.
Dave sat next to the Slytherins, the first first year to be sorted into that house. But he was quickly joined by another kid with a Mohawk.
"Name's Puckzilla. What's yours?" the kid said with an aggressiveness that almost seemed out of place in his tiny frame, but still worked due to the ferociousness coming from his eyes.
"I'm Dave Karofsky" Dave said, extending his hand.
The self named "Puckzilla" ignored it. "I have a feeling we're going to be good friends Karofsky" the kid said.
And Dave felt a knot in his stomach.
To say Puck was a bully was a bit of an understatement. He personally targeted and sought after kids who were struggling, and he made their lives hell. Dave never really thought it was right, but he didn't really want to leave his friends either. They were all he had at this school.
It took Dave a few years, but he got used to their bullying routine. They'd go after the kids who were alone, the kids who didn't have people to turn to. And they never did anything too serious. But still, nobody dared to mess with them. Ever.
After Puck joined the Quidditch team in second year, it became his priority to make all the other teams lose. But it wasn't working.
The Slytherin and Gryffindor teams had been in a struggle for several decades, each equally matched in both strength and playing ability. The Championship was always between the two houses, and nothing changed.
Until Blaine came to the school. The first year prodigy found his way into the Gryffindor team, and it didn't take Puck long to notice that Slytherin was losing because of the hobbit-sized first year. So it became Dave's job to make his life a hell. Every other day he followed Blaine down the halls, teasing him about his height, his hair, whatever came to mind. Sometimes Dave would back off because Blaine's friend would show up, but most of the time, Dave made him miserable.
But he stopped. One day in his fifth year, Dave felt a change in his disposition. He was with Puck this time when they met Blaine in the courtyard. Puck took care of threatening Blaine, telling him to throw the game, but out of nowhere another Gryffindor and a Ravenclaw. Finn Hudson was a generally known big guy, but Dave didn't recognize the smaller boy standing next to him.
But the kid named Kurt Hummel had enough pizzazz to convince Puck to back off. "Come on Karofsky, let's get out of here" Puck said, making sure Dave followed him. But Dave was thrown back.
No one had ever made Puck back off before, and Dave turned his head to see Kurt helping up the Gryffindor seeker from the ground.
Dave didn't really know what had changed in that moment, but it was something. Suddenly, he was less interested in bullying. From then on, Dave took a backseat to his group's tirades throughout the school.
Even getting on the train for his last year, Dave wasn't really sure why he was in this house. He had barely any similarities with the people involved, and he generally despised the time he spent with all of them. But he just kept his head down and mouth shut.
"Hey Karofsky! In here." Dave heard Puck's voice coming from a compartment. Inside sat Puck next to the girl everyone knew was his real best friend, Santana, and Brittany. Normally, Slytherin's didn't allow other houses to ride in their compartments. But nobody messed with Santana. Especially not when it involved Brittany.
"So man, how was your summer?" Puck asked, raising his eyebrows to demonstrate that, indeed, his summer had at least been entertaining.
Dave looked out the window and just said, "Oh you know. Same old same old".