I think, continuing with the rough thoughts I had above, that the important point is that adult murder was and is illegal, so there's no perceived aid lent to something that is still being carried out in the present without sanction.
It's difficult to come up with another example since killing has been at least de jure illegal through American history, but perhaps something like imagining that we are in an extraordinarily racist society where lynching an adult black person was and is legally permitted, and the cells were taken from that victim. That such evil acts were still occurring and could occur again for such reasons would make me hesitant to use a product derived from such.