It's interesting you put it like this because I interpreted the MSQ differently.
Hydaelyn, to me, seems like an aether leech at this point. As it stands, we have no confirmation (also correct me if wrong) that the lifestream as we know it existed before Hydaelyn sundered Zodiark and the Source. Upon doing so, the lifestream was created and all aether was separated between the 14 worlds. Hydaelyn seems to reside in the aetheral sea or "lifestream" as we know it. At this point, I am highly speculating that in liu of a lifestream a lot of the aether in the Amurotine's world was kept within it's people. It's huge 30ft people that can generate beings with their minds. That would take enormous amounts of aether/energy and they had it within themselves.
Getting back to the point and to put it bluntly, when you die, your aether goes to the lifestream where Hydaelyn sups on you for sustenance. Something has to sustain her or she'd cease to exist, no? Sure, you're inevitably reformed or reincarnated, perhaps even on the same shard as Emet-Selch points out being from the Source is very special and particular, but as it stands I don't really see a difference between mortals dying and going to aetherial sea and sacrifices to a primal. Either way, they are sustained off the aether of other beings.
Zodiark, at least from what has been revealed so far, appeared to require the sacrifices in order to fulfill the Amurotines wish to save their dying star. That makes sense to me as such a monumental task would require an insane amount of aether or energy and when the sacrifices were made he fulfilled his end. It was the Amurotines that chose to keep sacirificing to this primal, not the primal demanding it. They asked for things and Zodiark went "Well, give me the raw materials and I'll do it."
Because of this, I didn't see Zodiark as evil. He's...more like a computer program following tasks given. Hydaelyn could be the same perhaps. Her one rule being "Stop Zodiark from being used." Though she seems much more manupliative and and cunning in my opinion. It also concerns me greatly that she seemed to not care for the fate of the First until Minfilia was reabsorbed into the Lifestream. Hydaelyn either lacked the awareness or the capacity to care and this boggles me.
Either way, I will require a very compelling explaination from the MSQ to justify why Hydaelyn is a "good" being. At best I view her and Zodiark as tools that were misused, attempting to follow their individual protocol rules:
Zodiark: Ensure survival of star, Fulfill wishes of my summoners.
Hydaelyn: Stop Zodiark.
Anyway, if there are holes in my understanding please set them straight. I feel like I have a decent, if somewhat strangely interpreted handle on stuff.