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.
It's the characters and standard moral belief behind them, not so much the game itself. A test of morals.

We're presented with the head-scratcher that the world was essentially diluted into countless other shard/versions, as were all its inhabitants (who were originally an [almost] perfect race). We're then asked whether it is 'murder' to re-merge all the shattered people/worlds to reform the originals - a process that ends the existence of said shards/worlds which have been enjoying their own unique existence for ages post-sundering. Ultimately, it isn't much different to the what the sundering did in the first place - it reduced people from powerful immortals into flawed mortals with merely a fraction of their original 'stats' so to speak, but the difference is it was one group/world that was destroyed to create what we have now, so the reverse will be viewed as millions of lives being slain.

As expected, 99.99% of characters with any kind of intelligence post-sundering will not embrace this as acceptable in any way whatsoever, unless you're privy to that past in one way and/or convinced of the previous perfection (re. people like Varis, who was essentially raised by an Ascian anyway). After all, to rejoin them all and bring back the Ancients requires pretty much all of those shards/lives to be lost, one way or another, so you don't have to think too hard about how many would instantly reject it without any thought or consideration. It's the age old 'needs of the many/few' facing a vice-versa situation. Most people would likely say that the many > few in most cases by default. The Ascian's are striving for the opposite.

As such, the typical reaction is very predictable. "You want to murderize bazillions of lives? You must be one of those evil, filthy Ascians!"

Emet already mentioned how he feels about it. Having known what we were and seeing what we are, it's easy to see why he doesn't really consider such an act murder seeing as he wants to put everything back the way it was. It's understandable, but again, the characters (and most of the players) would still react typically. It's ending too much life in the present, thus it's bad, no questions asked. You must be stopped.