Megan McArdle I think has some interesting thoughts on the matter. Abortion is one of those topics where you just can’t win, so I don’t dare wade into it often. Who I am I to say where life begins? My skins cells are alive, and with the proper application of the right chemical compounds, at the right times, could turn into another one of me. Do I commit murder the shower each morning because each cell I scrub off is a potential me? But if human life isn’t defined at the cellular level, where do you draw the line?
I think Megan brings up some good points. If the line is drawn at conception, and a woman takes a morning after pill, should she have to face the possibility of having a needle stuck in her arm for premeditated murder? If the answer is no, then why? If the act of killing an abortion doctor isn’t defense of another, then why? I don’t condone the murder of Dr. Tiller, but I also don’t think abortion is infanticide.
Where does life begin? I don’t really have any special insight into that.  But I’m pretty sure judges don’t either. If abortion is to be regulated, then it should be done by the legislatures of the several states. But I’d be surprised if even the most conservative state legislature were willing to treat it as premeditated murder. That is why I suspect that the vast majority of people, even those who are pro-life, think murdering abortion doctors is wrong. But if that’s the case, can you still really say abortion is murder?