Ion said her power grew with her alliance with the Jailer (the head of the Maw). He's probably the first big end boss we face until we face whoever is behind him.
I think her motivation has been both her obsession and repulsion with death. She hates what was done to her and wants everything to pass into the veil beyond. The infusion of death power may have pushed her into it since her partner, the Jailer benefits from all these souls pouring into the Maw now.
Maybe, but that still doesn't really explain her change in motivation from the Before the Storm book - in which we actually SEE her motives/thoughts explained.
And while things can certainly change after the Calia's intervention and the Desolate council trying to broker peace with the humans, we see....literally no evidence of that.
Blizzard's insistence on keeping us in the dark is just silly at this point - though I imagine we'll get another book at some point to retcon the situation, it doesn't make it any less odd.
Sylvanas, pre-BfA, was protect the Forsaken, protect the horde, and not die/go back to the darkness that is her afterlife (as a result of being killed by frostmourne).
After/during BfA, that apparently changes to....twirl the evil mustache, fuck off from the horde/forsaken, and...rule (jointly or not) as a death goddess? (of which the last is kinda-sorta indicated by the three sisters comic)
Well if the leak is true and there is certain stuff that suggest it is the motivation of sylvanas is to gather souls because nzoth wants to free the other old gods.
Why would Sylv work with N'zoth tho?
Ashzara made a bargain to keep her people alive and 'stay' Queen - what similar benefit does Sylvanas get? Rule over a death realm?
I don't know if I buy that change in motivation as being so.....comedically 'evil overlord' of her - especially considering Blizz, at one time, hyped up her planning/forethought compared to Garrosh.
Of course...if she ends up a raid boss in a 'later' expansion, I suppose we don't....quite get Garrosh 2.0.
Someone stated on MMO-champ that she's empowering the jailor to make sure that the 'really bad stuff' stays locked in the shadow realm, i.e. hard woman making hard decisions, but I feel that's too close to what they pulled with Illidan to duplicate it so quickly.