Killberry being a branch of Blueberry instead of Swap is quite intentional, due to him being a remake of the infamous Yanberry. Killberry has only more recently become aware of resets. Maybe if he had told Papyrus sooner, he wouldn't have fallen for The Human's tricks and temptations. Much like how Killer has his tears of hate flowing from his eyes, Killberry has freckles made from the dust of the countless mosters he's killed in attempts to stop Chara. He will eventually leave his AU, being lead by Shattered and taking the role of Yanberry in the Fallen Stars.