You need to ask for consent before using hypnosis. If the girl says yes I assume everything is probably fine. If not, you run the risk of being arrested for using hypnosis on the unsuspecting, leading to a game over. I may or may not have learned this the hard way. I have no idea what actually triggers the arrest, I just had it happen to me.
Additionally, the yandere girl (Kylie) will also punish you for hypnotizing her (possibly regardless of her consent?) by growing increasingly more unstable and making it pretty hard for you to reduce her yandere stat.