The Sorting Hat Got It Wrong: Dumbledore Was a Slytherin All Along
Albus Dumbledore is a cornerstone character in the Harry Potter universe: a wise and powerful mage, the mightiest sentinel of Hogwarts, and the deadliest enemy of dark wizards, he's both famous and respected in magical and muggle worlds.
However, there have always been multiple theories about him being more of a manipulative grey figure — or a straight-up evil real deal. While this appears a bit too far-stretched, we can definitely see why he could've felt right at home in Slytherin and how weird it is that he ended up in Gryffindor.
On multiple occasions over the course of the books and movies, we are shown that Dumbledore knows much more about his students than he lets on.
He was the only professor who's never fallen for Tom Riddle's charm and deceit; somehow, Dumbledore always knew about his lies and real intentions. He seemed to know his students better than anyone, at all times.
The next generation that we know of included the Marauders, Severus, and Lilly. As we know, aside from Pettigrew, they all became Voldemort's most eager foes, and if we assume that Dumbledore influenced that, many pieces start to come together.
Initially, he might have been likely interested in Sirius, an heir to a Slytherin pureblood dynasty who rebelled and chose Gryffindor, and Severus, a grim boy who obsessed over both Dark Arts and Lilly Evans.
Dumbledore could see them both as his future double agents and kept them reasonably loyal by turning a blind eye to both the Marauder's reckless escapades and Snape's increasingly dangerous activities.
As a result, Albus did in fact receive a double agent and a bunch of fierce and loyal followers.
Later, he would be doing just the same with Harry and his classmates, and we can study his attitude toward his chosen students closer. He always stands by Harry, ignores his shenanigans, and even encourages him. In The Sorcerer's Stone, he literally provides Harry with the Invisibility Cloak, shows him the true meaning of the Erised Mirror, and effectively prepares him for the first encounter with Voldemort.
Since we know what happens in the Deathly Hallows, we can assume his logic here: it's a test run to see whether Harry will be worth it in the future. Even if he fails and Voldemort gets him, it's still a win — one Horcrux down.
Remember other chapters: many events follow this pattern, and Dumbledore is always above everyone, knowing and calculating it all. Now, do you really think that he was not a Slytherin?