'if a man takes a gun and shoots and kills a pregnant woman'...
Some people see a fetus as an unborn human, and everyone sees a pregnant woman as someone who should be protected, therefore the law (in the US only, I believe) accomodates this belief if somebody kills the mother and unborn baby they have killed two people.
This doesn't mean that there are others who do not share this belief, and it's a ridiculous argument against abortion. Many people also don't see a fetus as a human, and abortion is allowed, by law, to accomodate the beliefs of this very large portion of the community