A pregnant woman who is killed is ruled a double homicide. Thus, the termination of a pregnancy is the murder of a child.

No, because you're forgetting about the consent of the mother. If you kill the unborn child of a mother who wanted her pregnancy, then you are violating her consent to have that child, which is why it is wrong; whereas, if a woman chooses to have an abortion, it is because she does not consent to the child using her body for pregnancy, hence why the abortion is justified to end the violation of her bodily autonomy.