There is definitely a discrepancy here. But again, many people talk the talk but do not walk the walk.
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The discrepancy in your hypothetical question (and it is NOT inciting violence at all, we are merely conducting a peaceful fireside discussion)
comes from the doctrine of
self-defense. Everyone is permitted to take the necessary steps to defend oneself, and when the only available step is to kill the attacker - then so be it. No sane person will condemn the killing as “murder”. It is a justifiable homicide. Besides, murder is a legal term; it describes an illegal homicide.
Now the situation is a bit more complicated. Killing in defense of others is also permitted. No one would blame you if you used lethal force to protect your child, or family. Now there is the Christian concept of “love your neighbor as yourself”. Naturally it does not speak of your
literal neighbor, but about other people, in general. So it is permitted to defend your neighbor, too.
Of course there is a huge caveat here. Killing is only permitted if there is no other alternative. But when you see someone
about to perform an abortion, and you are absolutely convinced that there is no other way - then - based upon the doctrine above, you would be justified in preventive killing. What is the problem, then? Maybe, as you say, they do not really consider the zygotes and embryos as “people”. Maybe they understand that no judge or jury will accept their assertion that they used a justifiable force in the protection of “miniscule” people and they do not want to be thrown into prison. And with good reason. Just like
most people do not confuse eggs and chicken, or acorns and oak trees, they see the fundamental difference between a fertilized egg and a human being - even though under certain favorable circumstances the egg will grow and become a chicken, the acorn will grow and become an oak tree. As such society does not accept the idea that embryos are “people”, and the protesters understand the futility of fighting for a lost cause when the only outcome is losing their freedom in the process.
So they keep on talking the talk, but refusing to walk the walk. Pretty sensible, I would say.