1, The argument is that the fetus in the womb of a woman is a human fetus and that it is "a being, a living thing" and, that, therefore, it is "a human being." This makes sense only if one accepts the claim that it is "a being." The argument proceeds from the assumption that both points are irrefutable and that, when put together, they form a third irrefutable assertion. But, since it does not have the ability to achieve an independent existence, even with extensive assistance, the fetus is not necessarily "a being" in any sense in which the term "being" makes sense. The fetus has the potential to develop into a being. But, up to a certain point, it is merely a part of the mother.
If it could be shown that the fetus was "a being," then it would not be morally permissible to perform an abortion unless the life of the mother was threatened. In order to be "a being," the fetus must have developed to the point where, even with extensive assistance from a hospital, it could live on its own -- away from the mother. This is why there is a strong belief in a cutoff point after which abortions should not be allowed.
Even if one does not favor permitting abortions at any stage this argument is fallacious and does not prove what its makers intend to prove. It does not "prove" anything because its basic assertions do not hold up under the test of ordinary usage. In such usage, for example, a seed that has begun to germinate would not be called "a plant." This is the same distinction that we make about other animal beings as well and there is no reason not to make it here, other than the desire to make a distinction in order to say that a fetus is a human being.
2, Killing other human beings is almost always morally wrong with the possible exception of the defense of one's own or another's life. The argument asserts that no one can say for certain that a fetus is not a human being and we must, therefore, "err on the side of ca...