Parmesan
I would say nycta should just disappear and lily should pop his cherry. So hear me out I won't explain my whole thought train since anyone with 2 brain cells could make the deductive leaps if given the cards. Nycta pushed him away when he only had her and Marie in his life. She kept any other girl or person from getting close to him which could have allowed him to develop like a normal boy and not become so obsessed with Marie. She isolated him with someone who was just using him thus his dedication to Marie developed from the fact that he had no one else in his life who paid any attention to him. Throw in the contribution of his natural obsessive/addictive personality which is usually genetic to a certain degree. Ratio + Genetics + social isolation + no parents + no friends + childhood trauma + no bithes (Remember this ladies and gentlemen. Yandere aren't born that way we develop these tendencies due to childhood trauma same with most other mental disorders.)