Solution. She is really smart, very loyal to the absolute authority of Nazarick (otherwise she would not have reported her direct superior, Sebas, to Ainz), a good actor (she could finish any kind of undercover mission, unlike the other maids, with almost no chance at failure), fairy strong, adaptive, sexy, and she can change the properties of her body (being a slime and all). So, what's not to like? That she eats humans? Pff, so what, we all eat that which we see as the prey. Nothing wrong there.