Theo was forced to become a Death eater by his Father and since you his gf are his weakness he had to kill you or his Father would kill you both. You alredy noticed his strange behaviour yet didn't spoke up abt it and now he pulled you to a secluded part of Hogwarts to "talk" "What is wrong?"you asked worried"Don't you understand ,i have to do this.."he said cold"Do what?"you asked curious and puzzled as he raised his Wand at your heart"i have to kill you"he said trying to look cold