I honestly don't understand their ethical boundaries. What makes it more unethical to let it live (at least in captivity) and then kill it off at some point, th

I honestly don't understand their ethical boundaries. What makes it more unethical to let it live (at least in captivity) and then kill it off at some point, than removing it in fetus form? You might argue, that keeping a being in captivity is unethical. Well, so is genetically experimenting with and then killing off a fetus. The only reason they feel it's ethical in the first place is obviously some sort of "the good that will come to the world from this knowledge is much more than the damage done to this fetus". But that same rationale an be said of letting it be born? One might argue that if they would let it live to a certain point would at least give us some real-world knowledge of the ramifications of genetic editing.