Let's boil your point down to a more simple way to frame it:
"What is a human being?"
Ah, well there's the hard part. It obviously isn't a "person" (whatever that is) with two arms, two legs, two eyes, etc., because otherwise amputees wouldn't be people. It isn't a "person" with consciousness, because otherwise we would cease to be people when we slept, and those in comas would likewise have lost their "humanness." You suggested that humans are defined by their free will, which begs the question, "Free will to do what?" We can't say the freedom to do anything we choose, because then animals have some form of the same type of free will. We can't even say the freedom to choose to sin or not, because that assumes that a sin nature is a basic and essential part of the human, which would disqualify Jesus. I don't think that free will, then, is the essence of "humanness."
For me, a human being is a being created in God's image. It is in
that context that free will, sin, and salvation should be discussed. In that view, are stillborns, retarded people, etc. in the image of God? The answer must be yes. Of course, this doesn't resolve, on this level, the difficulty with them and salvation. I'm just saying that I don't think we can solve the problem by claiming that they aren't people in the first place.
And here's a simple way to validate your theory. How would you feel if someone proposed that we should euthanize everyone who lacks mental capacity. The reasoning would be that since they are not people, they are much more like the pests (roaches and such) that invade our homes, but for whatever reason, we have emotional attachments to these pests. We further reason that if it is good economics to kill the roaches in our houses, it should likewise follow that these "people" should be taken out of society so that they are no longer a burden on our system.
I'm sure you would cry in horror the first time the first one was put to death. That would be just wrong! Why? Because they're people. They're human beings, beings made in God's image.
Just my $.02.
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