The thing I will always respect about Blizzard is that this isn't childrens' lore.  They hire talented writers, such that their plots don't revolve around black&white dichotemies of good and evil factions.  Illidan is a complex character, just like Malfurion (who is a prick aswell).  Even Sargeras is a sympathetic character and he wants to destroy all being for fucks sake.

At the end of the day though, because of the nature of the game, they have to be either something we kill or something we don't kill.