1.
So, I think that Regulus should have been in Gryffindor, but he was too scared of what had happened to Sirius after his sorting, so he had begged the sorting hat to put him in Slytherin out of pure fear, the sorting hat, since it was created the way it was that it still had emotions like it's founders that it felt all of Regulus' emotions and put him in Slytherin for he thought that it would have been his fault if anything happened to the boy.
So that that!
Bonus:
Harry's second year when they take Dumbledore away from the school:
Dumbledore: "Help at Hogwarts will alway to given to those who ask for it."
Regulus in the afterlife: What a lie.
This is my first head canon or theory
Thank for reading ✨
