The point was she would be living in misery at the monastary and no longer have the power she had. Why kill her when you can have her suffer and basically watch you be with your partner happily for the rest of her life. With that girls personality she's sure to have a hard time there.
Oof he really shouldn't stop the poison training I know it's not great but better than him dying because he didnt have that training. It seems she a bit smothering. I hope she relents and let's it continue.