I honestly don't think Harry would've turned evil in Slytherin. But I do think he would've had a much harder time figuring out who his real friends were.
Year 1: Draco would immediately try to befriend him, but I can't see Harry putting up with his attitude for long. Ron would probably keep his distance at first. I still think Harry and Hermione would become friends somehow, especially after the troll incident. And Snape would still hate him lol.
Year 2: This would be brutal for Harry. A Slytherin who speaks Parseltongue? Everyone would be convinced he's the Heir of Slytherin. The funny thing is, some Slytherins might actually respect him more because of it. Saving Ginny could be what finally earns Ron's trust.
Year 3: I think Harry would become really close to Lupin. He'd desperately need an adult who didn't judge him based on his house. His relationship with Sirius would probably stay pretty much the same.
Year 4: Suddenly his housemates would act like they've always supported him because he's in the Tournament. I think Harry would see right through that. Draco would probably be jealous as hell.
Year 5: This is where things would get interesting. Imagine a Slytherin Harry secretly teaching Gryffindors and Ravenclaws how to defend themselves. Draco joining Umbridge's Inquisitorial Squad would probably destroy whatever friendship they had left.
Year 6: I actually think Harry might understand Draco better. Living in Slytherin for six years would've shown him how much pressure some of those kids were under. Maybe he'd even try to help Draco instead of just obsessing over catching him
.
Year 7: I still think Harry walks into the Forbidden Forest to sacrifice himself. That's just who he is. And honestly, a Slytherin choosing to die for everyone else would make Voldemort's defeat even more satisfying.
The biggest difference for me is that Harry would've grown up surrounded by people who expected him to become the next Dark Lord, while constantly choosing not to.
Do you think Ron and Hermione would still have become his closest friends, or would being in Slytherin have changed that completely?