(ShB Spoilers) Didn't Hydaelyn, uh...

[Lurina]

Technically yes.

But it was basically something wrong vs something wrong.
Sacrifying new lifes form to resurrect all the Ascian pre-Apocalypse would also have been the murder of billions of people.

I guess I have two things to say to this. First, this is very ends-justify-the-means logic. Even if we accept what Hydaelyn did as a ultimately necessary and positive act, we're talking about someone killing massive amounts of people as collateral damage in a bid to stop the villain. If this was any other person or faction save Hydaelyn herself, I think there'd be a lot more people who would be unable to permit it as a reasonable solution.

But second, if we do accept that a soul being sundered is death, Hydaelyn killed everyone in the world except for 3 people. No matter how big a sacrifice Zodiark had required, by definition it would be less murder than what actually transpired.

The sundered were not exactly murdered, simply rendered into 14 shards of themselves that all end up living new lives on an equally sharded world of their own. You can argue that this act certainly did end one life per sundered if you view it a certain way, but the reality is they weren't exactly "killed", thus murder is not the correct word. It'd be different if the sundering snuffed out each and every one of those lives. Instead, she essentially diluted lives into a glob of water and splattered it all over the place to exist all over the universe without any memory of the original droplet it started from.

Well, what I was trying to say in the OP was that the story itself seems to conceptualize this kind of messing with a person's essence as essentially murder by any other name. A rejoining causes someones soul to be merged with another (or rather, restored with the original broken soul), following which they live on as "part" of that person, if only in a spiritual regard. We see this happen with Ardbert.

The game, however, minces no words in defining this as murder when it is done by force, even though by your reasoning no complete termination has occurred.