Locked Up by My Mad Dog

Elizabeth found herself reincarnated as the deceased first love of Ernst, the male protagonist of a 19+ dark novel. And to make matters worse, she ends up with the fate of an extra who doesn't even appear in the original story, dying at the hands of the traitor Karsis on her wedding day. She tried to stop the rebellion to survive, but the scale of the mess the original owner of her body had caused was just too big. Not only had she had an inappropriate relationship with her loyal servant Karsis, but she had also wronged him terribly. Feeling guilty, she tried to set him free, but... "That won't do. Betrayal, that is. If you throw away a dog that was already abandoned once, this time it might bite its master." With half-crazed eyes, he was trying to kill me...? Wait, hold on. Ka-Karsis! (Source: Naver Corporation, translated)