Voidsent are pro at sniffing out people's darkest fears/desires and then tempting them into terrible deeds. Just remember what happened to the lady of Haukke Manor, who used to be a normal Elezen (and also what happens during the THM class quests). Voidsent are not just pawns for unhinged mages, but can also manipulate and warp a person in return.
Who knows what changed in Edda, either mentally or otherwise, because of her dealings with the voidsent. Whether that thing was really Avere or a creature feeding off her feelings/killings and lying to her -- who knows?