I don't think that this is an obviously crazy belief--I can see the argument for life beginning at conception. But ultimately I don't think it works, even for most people who profess it.
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.
I do not say such an outcome is particularly likely, although the more we know about fetal development, the more support for abortion seems to drop. But I don't think that it's particularly novel to note that our "instinctive" reaction to these things is partly, even largely, socially conditioned, not the product of deep rational thought.
We accept that when the law is powerless, people are entitled to kill in order to prevent other murders--had Tiller whipped out a gun at an elementary school, we would now be applauding his murderer's actions. In this case, the law was powerless because the law supported late-term abortions. Moreover, that law had been ruled outside the normal political process by the Supreme Court. If you think that someone is committing hundreds of gruesome murders a year, and that the law cannot touch him, what is the moral action? To shrug? Is that what you think of ordinary Germans who ignored Nazi crimes? Is it really much of an excuse to say that, well, most of your neighbors didn't seem to mind, so you concluded it must be all right? We are not morally required to obey an unjust law. In fact, when the death of innocents is involved, we are required to defy it.
As I say, I think their moral intuition is incorrect. The fact that conception and birth are the easiest bright lines to draw does not make either of them the correct one. Tiller's killer is a murderer, and whether or not he deserves the lengthy jail sentence he will get, society needs him in jail for its own protection.
Still, I am shocked to see so many liberals today saying that the correct response is, essentially, doubling down. Make the law more friendly to abortion! Show the fundies who's boss! You know what fixes terrorism? Bitch slap those bastards until they understand that we'll never compromise!
Well, it sure worked in Iraq. I think Afghanistan's going pretty well, too, right?
Using the political system to stomp on radicalized fringes does not seem to be very effective in getting them to eschew violence. In fact, it seems to be a very good way of getting more violence. Possibly because those fringes have often turned to violence precisely because they feel that the political process has been closed off to them.
We do not punish murderers by changing large sections of American law. We certainly don't punish them by, in essence, shouting "nya, nya, nya, we're killing more babies!!!!"* We punish murderers by sending them to jail, where they belong. If any of these changes to current law are justified, they're justified on their own merits, not because they'll piss off Tiller's nemesis.
* I understand that those advocating such changes do not perceive themselves to be saying this. But if you're trying to punish the gunman, and deter others, it's their perception that matters. And what bothers them is that they think you're killing more babies.