OK, let’s see
(1) Using 100%, “ruthless”, logic at the most fundamental level:
As you yourself have said,
" break it down far enough, and no morality can be logically justified: we just take some values as givens. If you think it is okay to torture and kill a person who screaming for you to please please stop, I can’t necessary present a fully justified logical arguement as to why it’s wrong,"
So, it’s hard to justify logically why we shouldn’t kill humans, let alone fetuses. So in this scenario, humans and fetuses are on the same level, i.e. cannot be logically protected from harm.
(2) Using the “invented” morality of human society:
Since this morality is invented based mostly on empathy and the golden rule (not based on strict logic), it is quite arbitrary what one society chooses to have empathy for, and what not.
In some societies, it may be considered OK to torture a kitten, but not considered OK in other societies. The choice is rather arbitrary.
Most people today would have a problem pulling the plug from a patient that is brain-dead but has some chance of coming out of the coma. This is quite arbirtary, since, while brain-dead he cannot feel any pain, and if he has no living relatives, nobody will feel pain from his death. So what if there is a small probability that he might wake up later?
In any case, when we use the invented morality of our society, we can arbitrarily add classes of beings we want to protect, and most of these choices cannot be challenged on the basis of fundamental logic.
So, some people want to add fetuses to the class of beings they want to protect. Quite arbitrary, to a degree, but, since we are under an invented morality that is to a large degree arbitrary, we can’t use logical arguments to prove that this is an unacceptable class of beings we want to protect. (Especially if this new class of beings shares something with existing classes of protected beings)
In summary:
At a very fundamental level it is hard to prove why anything should be protected, neither humans nor fetuses.
If we go away from that, we have to work within the framework of a morality system that is by definition invented and quite arbitrary. In this case, it is hard to argue against someone who chooses a morality system that includes protection of the fetus.