Because blizzard has given her an iron clad reason, to do what she did - to be revealed in Shadowlands. Something that you can relate to might have been necessary to possibly save or at least bring down the barriers between life and death.
IF you had the view of the after life Sylvanas had, with the urgency of the shadowlands problem - wouldn't you think entirely differently of the value of life and change your mind on whether to take it or not As a horde leader, would it not make sense if you needed more lives sent to the Maw, to rather take the lives of your enemies and thus drum up the war.
While this is never a good thing persay, do not the circumstances warrant a re-assessment of the justification of all this? What is life anyway? If passing into the shadowlands you just transition into another role? What does it mean to live this one here like that so briefly when eternity is where your main existence lies?
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Surely, then this life is only useful for determining what state you get in the next, and this life is where you get to choose. It's the old Christian argument on the afterlife. Where this life that you never chose to have, is given to you by God, however while you are here, you get to determine whether you spend eternity in heaven (and what degree or level or state you do so, doorkeeper or Elder etc) or in hell, tha talso has levels (the level is determined by your actions and choices in this life). Shadowlands is an alternate version of Christianity's message on after life.
The difference is that in wow, the afterlife seems more shades of hell rather than having any real heaven. But I reserve judgement until I actually see what the various covenants are.