I don't think that's true at all. If an infant is born completely braindead and permanently vegetative, do you think that life has a fundamental value that is morally indistinguishable from an infant who will grow up to perceive the living human experience? You might, but many people would disagree, and I think the crux of that disagreement is whether we believe in the existence of the soul.
That person certainly has fundamental value. I suppose you could argue over whether it's the same value as a "normal" person. But if, for instance, a pedophile were to use that infant to pleasure himself, you would certainly agree that is disgusting and wrong (and you can sit here and try to claim otherwise for the sake of winning the Internet sperg argument, but I won't believe you for a second); even though if we stipulate that the braindead infant somehow isn't actually a person due to being braindead, this act would then be equivalent to using an inanimate sex toy.
The term "soul" is just shorthand for the idea that human life has inherent value. Again, atheists obviously share this value, even though like you are doing, they sometimes try to claim they don't. But revealed preferences (i.e. actual behavior vs lying for the sake of politics) always betray this claim.
If we're going to go into extreme hypothetical, I do believe nothing is wrong with instantly ceasing somebody's life in a way that adds no pain, grief, suffering or inconvenience to anybody else left in the world. Everybody dies, and I don't believe that the dead know or care that they're dead. Unless they have a soul that is being cheated out of further human experience that God intended for it, I'm failing to understand what you think the damage is.
It's not an extreme hypothetical when it's the obvious direct implication of your stated belief. Calling it "extreme" implies that you don't actually believe what you say do, because if you did, then there's nothing "extreme" about it. I don't believe your answer because we both know that if you were put into this situation yourself, you wouldn't pull the trigger. Although we are seeing some consequences from this sort of mindset where you try to pretend that life is meaningless, namely the expansion of euthanasia and the rising possibility that the government will encourage the old and the poor to simply dispose of themselves. If you really want an extreme hypothetical, here you go:
An alien race kidnaps you and puts you into a chamber with two buttons. The aliens have a giant death laser pointed at the Earth from an extreme distance--no one on Earth has any idea that they have it, or that the aliens even exist. The aliens tell you that you have one minute to press one of the two buttons, after which they will execute you in a painless fashion. If you don't press one of them, they will torture you relentlessly in unimaginable ways. One button will fire the death laser, instantly destroying the Earth and all of humanity with zero awareness or pain for any of them--just boom, gone. The other button disables the death laser. The aliens don't care which one you press, won't interfere with Earth further one way or the other, and are doing this for the lulz.
Which button do you push?
My basis to condemn Aztec sacrifice is obviously that it caused a lot of pain and suffering, and even more obviously, that I don't believe that the sun will stop coming up if we don't cut people's heads off. If I believed that, I'd probably be okay with it because the sun coming up is pretty important to me. If I believed in a soul and in God having a plan for all of them, I probably wouldn't be okay with abortion.
We could theorize a situation in which the sacrifice is being done painlessly, which actually isn't all that different from some modern practices, such as people who decide not to have children in order to somehow save the climate. But what I'd rather ask here is why pain and suffering matter to you if human life itself doesn't matter. Why do you care if a human suffers, if you don't even care whether they live or not? Just let them be tortured for someone else's amusement. There is no God looking down and judging you, no true morality that could possibly be violated, after all; everything is just pointless particles that exist for no particular reason, and none of it matters.