I think it was wrong for Baine to give Derek to the Alliance, as it was a clear security risk. At the same time, Derek shouldn't have been enslaved (as it pretty much breaks the one rule regarding undead is that they can do what they want as long as they don't threaten the Forsaken), it should have been "Hey, join the Horde and continue your unlife, or die a second time." Because, if Sylvanas will do it to one raised Alliance person, who is to say she won't do it to others? Thomas Zelling for one was perfectly willing to be loyal to the Horde until the situation with Derek. Whose to say that, with positive reinforcement instead of torture, Derek would have willingly join the Horde. A long shot, sure, but still.