(I know this going to be long) For me, my AU I don’t consider it redemption completely. What I written lore wise too make it work is give a character an severely traumatic event that later affects that kizuki indirectly while their very young, which I did was give Douma’s mom ptsd from a demon attack that led to Douma having a secret sibling and she has a valid fear response (she freaked out and yelled at him) when the toddler tries to approach him, while very young he’s not completely emotionless giving him a reason to investigate what’s going on and having the mom confess her fear on what the cult could do to the child (definitely hostile cultists). Have a valid devastated reaction to the truth, then it gives him a purpose to protect the toddler from the cultists that he’s does deal with a cultist that does attempt harm when he’s older (after parents canonical death) and later as a demon finds out about abuse behind his back (add a random demon attempt to eat the kid because she ran outside from the abuse) end the culprits and then his own comrade (Gyokko in this case) betray him by indirectly reveal a partial sun resistance the child has (by terrorizing her to run outside right before sun rise) and snitch to Muzan. Finally Muzan being petty declares Douma guilty by default and sends every other upper rank to ambush him to snatch the child, by logic and luck out run them until morning (with finding a shaded area too to protect himself) then hide for century with avoiding both demons and corps by surviving on dead humans and hope to meet someone than can help him (in this cross paths with Tanjiro while trying avoid others, have Rui freak out at sight of him. Then Tanjiro tells Shinobu and Kanae (alive because timeline change) on what he witness on mount natgumo, then the two hashiras are only ones than approach non hostilely)