Argonis said:
I get the feeling that she would snap and go [ if i can't have you then no one can]
and she would rip your head off then go to some deserted island to be alone with your severed head.
No, because I'd make it perfectly clear that she CAN have me, so long as she obeys me unconditionally. That, and I'd make a theatrical rant about how if she did do something like that, I'd hate her from the afterlife, if there is one, so much that when she gets there I'll be sure to express my love for everyone but her.
I think that, facing the threat of complete and utter rejection or the promise of what they want, a yandere could be made to cave. Easily and completely.