He took the Morals but not the Free Will in otherwords. The Darkness in Sylvanas’s Soul promptly capitalized on the removed Morals and grew to fill the Void inside her.
Arthas also had his Soul drained but it was not the Morals that were taken from him but the Immorality which the Darkness that rests inside all Souls capitalized on to grow to fill the Void. As you can guess the Darkness was less interested in Self-Righteously Justifying itself and more in claiming power.
Incidentally the constant draining of Arthas’s Soul prevented the Darkness from bonding with Arthas’s Morals so it became a separate Personality that the last vestiges of Arthas’s Soul gnawed at until it got shoved into Frostmourne!
Incidentally when Lysonia got Uther(the source of the Forsworn’s Domination Magic) to start shoving his Soul’s Domination Energies into the Paragon of Humility his Morals started coming back. In otherwords both Darkness and Domination was in Uther’s Soul and he managed to get rid of the Darkness(but not the Domination) only for Lysonia to be consumed by it!
Lysonia also had a chunk of her Soul ripped out of her by the Paragon of Purity just to extract the Memories tied to her becoming Forsworn. End result: She got consumed by both her Darkness and Uther’s Darkness!
Lysonia overreached when she got Uther to start pouring his corruption into the Paragon of Humility at full blast just to access the Pride he doesn’t have. Trying to subvert the Paragons to their opposite was Lysonia’s mistake as she should have been capitalizing on the Vices that complement their Virtues. The Paragon of Humility’s most obvious Vice would be Fear. Fear that Lysonia refused to capitalize on!
Of course the fact that Uther was able to expend the Darkness before he expended the Domination Magic shows that Darkness in the Soul is not infinite despite growing with the size of the hole in one’s Soul.
Uther had us to help him reunite with his Soul Fragment but Sylvanas just had her Soul Fragment abruptly shoved in! She might have multiple personalities!
Of course extracting the Banshee Queen(with a Mourneblade in all likelihood) might simply leave a bigger hole for the Darkness to capitalize on and without subtlety(probably giving us the 7th Cosmic Force’s true face and voice(inb4 someone says it’s Amon from StarCraft)) forcing us to figure out how to extract the Darkness into the Mourneblade.