From the time Harry Potter wore the Sorting Hat, there was actually a chance that he might have ended up in Slytherin. The Hat had picked up on his ambition, cleverness, and ability for greatness—characteristics that would have fit the House to a T. Yet Harry pleaded not to be sorted into it, and he was sorted into Gryffindor instead.
As a Gryffindor student, he personified the house values such as bravery, courage, chivalry, and determination. In a parallel universe, what if this had not been the case?
Harry Potter’s Friendships and Rivalries Would Have Changed

If Harry had been in Slytherin, his clique would have been completely different. He probably would not have grown close to Ron and Hermione since the Gryffindor-Slytherin rivalry would have prevented their friendship. The Slytherins looked down upon them either way, since Ron was not from an elite family of wizards, instead, another in the huge Weasley clan, and Hermione was worse: a muggle.
So, Harry might have befriended Draco Malfoy and other Slytherins, but their group dynamic could have been complicated. Although Malfoy had initially looked up to Harry, their friendship would have rested on whether Harry embraced the Slytherin principle of blood purity—something which he would have rejected.
He would have also played Quidditch clad in Slytherin’s green and maybe had a tussle over the seeker position with Draco. The team’s corrupt ways might’ve actually ruined Harry’s reputation and peace too.
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Harry would also have been under even more suspicion by Dumbledore and the Order of the Phoenix. A Slytherin and a Parselmouth, he could have been seen as a future dark wizard instead of a hero. This would have further isolated him, and he would have had to depend on his own wit instead of guidance from others.
Would Harry Have Become Darker?

One of the largest issues regarding Harry being a Slytherin is whether or not it would have steered him towards the dark side. Slytherins are not necessarily evil, but being among the students who were so enamored with Voldemort may have had some effect on him. Nevertheless, Harry’s sense of justice and compassion would have probably kept him from the dark side, considering he was the only one who could end the Dark Lord’s reign of terror.
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All the same, he could have tackled difficulties differently. Rather than rushing headlong into danger as a Gryffindor would do, he could have been more devious and calculating, employing guile to defeat his foes. His battles with Voldemort would have been games of the mind rather than brawn, and he could have been an even worse foe.
Possibly, Professor Horace Slughorn best explained the characteristics of the Hogwarts house, which might’ve chalked out Harry’s journey too: “Slytherins are often misunderstood. We have a drive to succeed, to prove ourselves, which can be easily misconstrued as an evil trait. But ambition is not the same as wickedness.”
Harry in Slytherin would have altered the whole story, resulting in darker, more complicated conflicts and a very different wizarding world.