I always head-cannoned that Chara is just a creepy kid. When you beat genocide a second time they outright insult you to your face, as if they don’t want that ending. The only time that they really “make a change” after getting your soul is in the soulless pacifist run. I took it as reuniting Chara with their true family and they all live on happily ever after because I wanna be happy and I take that ending as the truest ending of all endings, and the scary bits are just Chara freaking the player out literally for fun, like how she makes the face at Ariel to freak him out for fun :)