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But then, there was the epiphany that some people were into playing evil races/factions. Whereas a LOT more people wanted to play as these "evil races (Forsaken not included) but as the good guys (Thrall’s Orcs during WCIII).
So, Metzen needed to come up with something for the undead. We seriously considered dropping them, but every time we looked at concepts/in game assets we balked. Then he announced the Forsaken.
How else would you interpret it? During the development of WoW Blizzard recognized the demand of players wanting to play an evil race but be the good guys about it. They struggled with the concept of playable undead, Metzen made the Forsaken to that demand.