Why are people surprised that the Duke is bad and wants to keep Ripley against her will ? Him being a "crazy self-absorbed psycho" (and I'm quoting Ripley) is how he was always presented! He's a yandere, what were ya'll expecting ?
Ripley too, has big flaws. She lacks honesty, self-control and common sense. But regarding her actions toward the duke, you're being too harsh. He's a yandere. Yanderes are dangerous (not to mention her trauma & will to see the true ending).