Once again, semantics plays a big role in the debate about abortion as murder. I think it is possible to make reasonable arguments both for and against abortion as murder, but I also agree to some extent with elef3u that the very lack of agreement on this point informs how we should structure our laws regarding it.
Murder-How about calling it the killing of a human being without a justification acceptable in the culture at large? (this leaves plenty of room for self-defense, warfare, state-sponsored execution, victim’s consent, and euthanasia, all of which are, of course, ethically debatable in their own right and would be classed as murder by some). This begs the question of majority and minority rights (see below), but is realistically how most of us use the word.
Then the debate shifts to “human being.” The boundaries of exclusion for this term are arbitrary and, if most cases, not especially logical. The scientific definition of species (individuals capable of producing fertile offspring) isn’t very useful here (legalizing killing those people with chromosomal abberations preventing fertility wouldn’t get much support under any world view). Cognitive criteria leave out those with brain damage, dementia, and many other disorders, not to mention the unborn, and frankly could reasonably include individuals of some other species as “human.” Birth is a traditional point in many societies, though some cultures have not endowed individuals with personhood under law or custom until 1-2 years of age. How about viability outside the womb, as the Supreme Court decided? Well, technology changes that boundary all the time.
All of this is to preface the idea that one can make a logical (not the same thing as reasonable nor correct) argument for abortion as murder simply by deciding to label as a “human being” the fetus at whatever arbitrary point they chose. Conception is no less arbitrary than birth.
Personally, I don’t like calling a bunch of nearly identical cells a person since it is absurd on the face of it and leads to the “every sperm is sacred” nonsense. If it were up to me, I’d pick a dividing line somewhere around the point where some reasonable number of fetuses on average (say 75%) could survive without the need for advanced life-support, but that’s just a practical solution that’s not likely to make anybody happy.
As in most things, a well though out determination will admit that clear boundaries are meaningless and that any dividing line is somewhat arbitrary. The belief system underlying one’s values creates the context of the definitions, and these then determine the ethical foundation for a final decision. The best we can hope for in the absence of widely-shared consensus on the underlying beliefs or definitions, is a mechanism for making the decision that minimally infringes on the individual’s freedom. Forcing someone to do something major with their body (have or not have a child) and with their life ever after (such as raise a child) is an immense burden, and given the lack of consensus and of objective, agreed-upon criteria for making the decision about the ethics of abortion, we should be very leary of compelling individuals in this area. In this sense, even though the general idea that the “majority” should get to dictate what everyone does is ethically questionable, it is true that an extremely broad consensus should be required before individuals are significantly burdened in terms of their personal freedom. As I said above, I do think it is sometimes appropriate to impose such burdens or compulsion, and I don’t have a general answer applicable in all cases as to when it is or isn’t appropriate nor what constitutes a sufficient majority, but I would err on the side of individual freedom whenever there is substantial disagreement, as with abortion.