I find it hard to argue with Jacob Sullum's logic:
Yet if you honestly believe abortion is the murder of helpless children, it's hard to see why using deadly force against those who carry it out is immoral, especially since the government refuses to act. It may be unwise or counterproductive to the cause, as Schenck suggests when he worries that the killing could be "a greater setback to the pro-life movement than anything the so-called pro-choice movement could do." Promoting an image of pro-life activists as murderous extremists might dim the prospects for legislation restricting abortion, thereby leading to more deaths of unborn children than eliminating one abortionist prevents. But this is a tactical question that does not have to do with the inherent morality of killing in defense of innocent children.
Nor is it sufficient to note that killing Tiller was against the law. When the law blesses the murder of babies, it is hardly worthy of respect, any more than laws blessing the enslavement of Africans or the gassing of Jews were, and violent resistance against such enactments surely is justified in principle.
Some of Sullum's commentors point to a third option: pacifism, which might decry abortion as murder, but morally forbid violence in response. Others discuss the nuances of Just War Theory as it would relate to an extrajudicial killing.
UPDATE: Further ruminations from the ever-thoughtful Megan McArdle.
So. Now I can move onto the observation that if you actually think late-term abortion is murder, then the murder of Dr. Tiller makes total sense. Putting up touching anecdotes about people he's helped find adoptions, etc, doesn't change the fact that if you think late-term abortions are murder, the man was systematically butchering hundreds of human beings a year--indeed, not merely butchering them, but vivisecting them without anaesthetic. I'm sure many mass murderers have done any number of kind things over the course of their lives, to which the correct response, if you're trying to stop the murders, is "so?"
Imagine a future in which the moral consensus has changed, and our grandchildren regard abortion the way we regard slavery. Who will the hero of history be: Tiller, or his murderer? At the very least, they'll be conflicted, the way we are about John Brown.