By Dreknar's logic, what Arthas and the Lich King did were perfectly morally justified, in that case. Still, we were perfectly able to justify killing off the Scourge. Go figure.
Only reason the Forsaken still are allowed to be around are because they are a part of the Horde. If they were not, we could bet our asses that Sylvanas and co. would be next in line on the boss menu.