For those of us with a libertarian bent (or an interest in libertarian views), I recently encountered a compelling argument based on the non-aggression principle which both (1) presumes that abortion is categorically the killing of a human being, and (2) endorses a pro-choice position. Block and Whitehead, "Compromising the Uncompromisable: A Private Property Rights Approach to Resolving the Abortion Controversy" (PDF).

Essentially, the argument goes:

• A woman's womb is her property
• An unwanted fetus is therefore a trespasser on her property (even if an unwilling one)
• Therefore, the woman is justified in killing the fetus if that is the only viable way to remove it from her property, just as a landowner would be justified in using violent force to remove a trespasser if and only if all nonviolent attempts had failed.