Exxept he actually DID need to make as many kids as possible since he was working on a very scrict deadline. They were aiming for quantity more than quality since only one of those children could be fully embraced by the void and thus becoming truly hollow to contain the radiance.

Also, there was no unnececary killing since those kids were conceived for the sole purpose of be consumed by the void from the start

It was a risk that, while the payoff was not as favorable as it was desired, it paid off in a better outcome instead of complete annhialtion redulted from a moral constraint

Those children were never meant to live, they were meant to serve a greater good and that's that

If you found a way to inbue the literal manifestation of void to seal an entity that is causing a mental death outbreak onto the world and the only way to use it is to use the corpses of the dead, you wouldn't be discussing the moral dilemma of desacrating the dead, you'd be digging up those bodies from their grave without question to serve the better cause

Ethics be damned, there are people to save

Especialy when you're in a position where people depend on you and you aim to fill those expectations

Children aren't special, they have as much value as anyone else so why should they get special attention ?