I know if I had a child who was born with a horrible affliction and who could never make a life for themselves and be happy, I'd rather it not have to suffer at all.
Think a few links deep into human psychology. It never occurred to me either.
If human beings are just chunks of meat with brains, and we can kill one, we're not "100% safe."
Most people have low self-confidence and want to escape natural selection, so they want laws and rules and taboos so they are "100% safe."
You and I and most people on this forum know that's a fucking illusion... but that's why they demand such egalitarian views.
Then shrewd clever but not intelligent people pander to that, and that's how we get the dogma extended to retards, the third world, minorities, and others who really don't fit in a civilization.
I say laugh and move on. Too much lament ruins a civilization. Better to say, "well, that didn't work," and return the soul to the cosmos so it can create new creatures with it.