That's beside the point. Murder is prohibited in the Bible going right back to Cain and Abel. The prohibition isn't predicated on the existence of the soul, but on human beings being the image-bearers of God.
Embryos are human beings. Therefore, they bear the likeness of God in some sense, and it would be a grave evil to destroy them--whether by murdering a pregnant woman, dismembering them in an abortion, or disposing of them because they are surplus to IVF requirements.
What the unborn is, determines what we may do with it. If they aren't human beings, no justification is needed for destroying them. If they are, no justification is possible.