I have to be blunt and honest: I have mixed feelings about the murder of George Tiller.
I am, of course, 100% unequivocally pro-choice. I have no mixed feelings whatsoever about Tiller or Scott Roeder, his murderer: Tiller is a hero, and Roeder is a villain.
The issue is why I praise one and condemn the other. I approve of Tiller because, and only because he's pro-choice (I admire him as well because he did something about it, at great personal risk.) I condemn Roeder only because he's pro-uterus-slavery.
Many ethical conflicts can be resolved peacefully and to most everyone's satisfaction by just looking at the issue at a more abstract level. Some conflicts cannot be so easily resolved; they can be resolved only by one side coercing the other. In ordinary life, we do not try to find an abstract common ground and mutual satisfaction with common murderers and rapists: we simply use the raw power of the state to coerce them into compliance, and kill them if they refuse to be more gracefully coerced by arrest and prison.
In larger issues, the ethical and political conflict between the feudal aristocracy and the bourgeoisie could be resolved only by violent revolution, including the American revolution. The ethical and political conflict between slave-based semi-feudal agriculturalism and industrial capitalism could be resolved only by the Civil War. (Note that European countries that eliminated slavery relatively peacefully did not have the economic entrenchment of slavery in the American South.) The conflict between semi-feudal imperialist-submissive Russia and China and communism could be resolved only by violent revolutions.
It should be relatively obvious that the pro-uterus-slavery movement is a revolutionary movement. (More precisely, the pro-uterus-slavery movement is part of a larger right-wing reactionary/fascist revolutionary movement, intent on destroying not only socialism but capitalism and bourgeois democracy as well.) It is a matter of objective truth that if you wish to make deep changes to society, sooner or later you'll have to fight a violent revolution.
As a revolutionary communist, I can't deprecate the idea of revolution itself; I can only praise or condemn revolutionary movements on the basis of their content. I'm against the pro-uterus-slavery revolutionary movement and the larger fascist reactionary movement because I'm against uterus-slavery and against fascism.
We do of course need philosophical argument, but philosophical argument alone has never led to substantial local change, much less the larger changes of social, political and economic revolutions. No successful capitalist ever read Marx and said to himself, "Hmm... Maybe I should stop exploiting the masses." No successful feudal monarch ever read John Stuart Mill and thought, "Hmm... Maybe I should grant unrestricted freedom of speech." To effect change, philosophical arguments must be combined with the will to act to effect changes in society, and those attempting change must presume the violent resistance of those benefiting from the status quo.
I'm not going to analyze Roeder's infantile tactics; the fascists can read Lenin and Mao on their own if they want to understand effective revolutionary strategy and tactics. I will observe only that criticizing Roeder's tactics misses the point of the fundamental ethical issue: Roeder is a villain not because he murdered George Tiller; the murder itself is only bad revolutionary tactics. Roeder is a villain because — and only because — he's for uterus-slavery. Indeed everyone who is for uterus-slavery is a villain not because it somehow "leads to" murder, but because uterus-slavery is itself abhorrent.