This is the ‘not being so black and white’ part.
The Alliance quit seeing the Forsaken as the Lordaeronian people that they originally were before their deaths.
The Forsaken aren’t “claiming the ashes of Lordaeron”, rather, they’re retaking what was originally theirs to begin with back when they were still alive and protect it from those who now want to take it from them just because they’re undead now.
Imagine being a human living in Lordaeron before its fall. One moment you live a happy life, you’re proud of your kingdom, you have a family, a business and everything’s great.
One day you wake up and everybody and everything that you’ve ever known and loved is dead or destroyed, Your home is gone, you yourself have lost most if not all of your humanity and your former allies (the Alliance, Gilneas, The Scarlet Crusade, The Kirin Tor) are now trying to hunt you down and exterminate you and all of your kin who have become “free-minded” undead just because of what you are now.
(Lillian Voss’s story is a good example of this)
In all of that darkness and confusion, there’s one person who reaches out to you: Sylvanas.
She gives you clarity, understanding and most important of all: Hope and a cause to fight for.
That cause being to retake everything that was taken from you, rebuilt all that was destroyed and to get vengeance on those who did this to you.
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If anything The Forsaken are one of, if not the most tragic race of WoW as well as being the most enduring one.
They’re victims and survivors, they most certainly are not evil.