Because he wants Sam out hunting. Sam's the perfect general because he already comes pre-trained right out of his pretty, pretty packaging. YED probably figured that if Sam saw his threat (what he was gonna do to her if Sam stayed) he'd brain up and go.

Then Sam came back and planned on STAYING and that was just NOT going to happen because he would've gotten even softer than he was already, so YED burned her, effectively sending Sammy back out hunting again and working those muscles of his.