I'd say that's presupposing a frame, or in this case, trying to apply real-world human morality to a society with skills and abilities that we can't readily comprehend on a moral level. The easiest analogy would be human beings and our most prolific creation: machines, which we routinely use to the breaking point, abuse, or destroy whenever necessary (or whenever we feel like it) because we understand they're automatons completely lacking in sentience or sapience. To beings who are capable of creating life as a mere thought exercise, they may view their creations the same way we view machines - even though they are fully sentient, they're categorically lesser beings who owe all to their creators and for whom any outcome is manifestly justified because their creators are essentially gods. This notion maps well onto the Ascians because beings like Emet-Selch have already betrayed that sentiment - they don't see beings like the Warrior of Light, the Scions, or anyone else inhabiting the sundered aspects of Etheirys as actually real, just shadows or phantoms of real beings, to be moved or abused as is necessary for their ultimate goals.
The pre-sundered Ascian culture probably viewed their own creations in a similar light, just phantoms of "true life" (the kind of life they themselves represented), so just evaporating a created being because it didn't pan out properly wasn't considered heinous or abhorrent to them.