It is puzzling that so many libertarian thinkers get so worked up if you suggest the obvious fact that abortion is, you know, killing, and that the fetus has not been liberated from anything but life. (There are, of course, refreshing exceptions to this rule.)
The reason, I believe, is the issue of obligation: If abortion really is wrong, that implies that one could have an obligation to care for one of one's fellow beings into which one has not voluntarily contracted. Denying that any such obligation can exist is part of establishing the reign of the ego over reality, of the ego installing itself as Supreme Being. Yes, the libertarian ego realizes it must compromise with other egos that occupy bodies out and walking around in the world, since they might have guns and could kill one. But fetuses can't kill you! If they become a barrier to the ego's desires they can be chopped up at your pleasure.