That the Void Elves are no more traitors or a danger than any other magic user in Silvermoon, and there was no reason for them to be exiled when any form of magic used without proper research and supervision would be dangerous to a massive font of energy. Particularly Fel magic, which is more pervasively corruptive, more unstable and more explosive.They just created more of a risk by kicking them out of the city to a place where they had few resources and no supervision, and subsequently left themselves completely devoid of knowledge of the Void and how to deal with it.