The problem arises, with humanizing a human. If we accept the perspective that we are just organic patterns that self perpetuate over time, then terminating a fetus is no problem. Murder wouldn't be a problem either except, people have developed an ability to feel pain, despair, and fear. Constructing morals on top of a sociological need for self preservation and actual growth (emotionally as well as physically) is the driving force behind law.
If a fetus has no soul, it really is nothing but organic material, with potential to be self sufficient organic material. But quite frankly we don't even know humans in general have souls.