By the same logic used here, you could say various rocks are pretty valuable because they will eventually turn into gems - and while that is mostly true, it's not a valid argument for anything. More extreme case, murder could be forgiven alltogether, seeing how human isn't much more than a ground fertilizer -eventually-. So no, you cannot treat something that is not human by any standards anyone can set, because there is a high chance that it could turn into human -eventually- when left alone. There is a reason why we treat things depending on what they are, not based on what will (likely) they become in the future.
You keep making essentially the same comparisons over and over, but just changing the individual object, and I don't think your examples are really applicable. A rock is not a human being. Why would anyone care whether a rock might turn into a gem in a million years? It's not like anyone is going to be around to harvest it if it did become a gem. If they were, maybe they would care. Pro-lifers see an
inherent value in human life. I'm thoroughly Atheistic, I don't believe in any sort of divinity at all, but I can appreciate this point. At least in regards to the beginning of life, while innocence still exists.
If you choose to believe that an embryo is not human (kind of silly IMO) or not a person (a more reasonable stance, not one that I think is relevant, but certainly more reasonable), that's fine. It's not going to convince a pro-life person that the embryo is any less special, though. You keep arguing as if your stance is rooted in some sort of absolute inescapable logic, but on both sides of the issue, it's really a question of two different, incompatible moralities. Edit - if you don't see an inherent value in the life of the unborn, that's fine, but recognize it for what it is -- a moral decision, not something grounded in science or logic.
Murdering an adult is a whole different discussion. Executing an adult who has committed a crime, also a different situation. Context is everything.
Also, this is not a justificaton, this is a fact. Embryo is not a human and / or a person with rights it is a mass of cells and only mass of cells unless you apply spiritual or religious meanings, which is what is happening here.
I'm not arguing that an embryo has 'rights'. 'Rights' are arbitrary, an intellectual construct intended as framework so we can all try to get along without killing each other and pillaging and looting everything in sight. In my book, the state decides what rights we all have, and they are subject to change under the rule of law. If the state rules that an embryo has no rights until it reaches a certain age, then it has no rights. There's only one truly fundamental characteristic (I hesitate to call it a 'right') that we all have -- free will.
The flaw I see in what you say is very clear in your summarisation at the end. "Here is a thing that will eventually become a person, and what do you want to do about it?" - Wrong. That is a false, flawed statement. The thing will not become is person nor is one. The thing, given time, nourishment and several other factors has a certain chance of becoming a human. You may say that is getting close to semantics in a way, but it is important. "You will be fired." and "You will be fired , should you violate agreement stated in your contract." - do you see the difference here?
I don't get what you're trying to say here. The mother doesn't have to
do anything in order for an embryo to develop into a fetus. All she has to do is to not die. It's in her own self-interest to stay reasonably healthy, and it's no coincidence that her good health also affects the good health of the fetus.