He who was is infected with the Shadowcurse,which means, that he is doomed. There doesn't seem to be a cure.
I did understand this as the reason he did go against the Ravenqueens order. When you kill him, you find a letter from a fellow Shaddar Kai expressing concern for his condition.
Of course trying to save him could be a questline.
I could also see Roah Moonglow as an evil companion. Since she is a bard, that would fill out a class slot, that isn't already taken, if Tav isn't a bard. As a Zhentarim her recruitment could be tied to how one I reacts in the missing shipment quest and you of course have to destroy Nine Fingers guild.
Then there is Yurgir, who is very much balanced, when he joins you as an ally later on.
