You're all forgetting that Avere was already long dead. Most likely, a voidkin (floating eye) was simply playing on Edda's weaknesses to gather more souls to consume/become stronger/whatever. The fact is that the boss is NOT Avere. It is simply NOT Avere.

As to the why, think about it: why kill one helpless girl when you can get her to lure others in at a "rescue attempt," only to be consumed, further fueling your strength? (You should all read Steve Harvey's new book - Act Like An Adventurer, Think Like A Voidkin).