Damn, a lot of you lack some fairly basic reading comprehension. It literally told you, through Orba's words and the flashbacky-panel of her during the war, that this empty village and these fields of her (at least at this point to her) presumably dead father are her motivations that carried her through a whole-ass war (against scary ass demons too!). Top that with the fact that she probably wants to avoid fighting other humans with her daughter-turned-sword... It's easy to see why emi didnt fight there.
Top it off with all we've seen recently of how her hatred for maou and the demons is wavering, it's pretty easy to see why she wouldn't be in a good headspace. She could have easily wiped the floor with them physically, but mentally/emotionally she wasn't up to it.
Still a little dumb and contrived to make maou go back to isle ente, but it's not that bad...