Given that I have never and will never throw "monster's" children into the mix, and times and opinions have changed over the past half century, I think sometimes morality depends on assumptions and settings. I assume most people wouldn't think twice about killing a swarm of demonic Dretches or Larva. But those are basically the equivalent of fiendish children. People will say that that's okay because these are formed from the souls of evil individuals or born of the chaos and evil of the plane.
But what if my campaign has reincarnation? I have slurgs, an evil race that people who were particularly evil come back as because my campaign doesn't have other planes of existence like Avernus or The Abyss because I only have the prime material. They're born evil and malevolent with no chance of redemption. They are fiends with different fiction and lore. Is it always evil to kill them before they have a chance to harm others?
Like I said, I'll never throw the evil children scenario, even if those children are irredeemable. But I also won't tell people they're wrong if their setting assumption includes always evil monsters. Some people like simple black and white campaigns with clear lines. I don't judge others based on how they play their games, I certainly won't judge a mod written over four decades ago.
