I was reminded today of John Piper's 1995 sermon, "Fasting for the Safety of the Little Ones":
This war will not be won by bullets. It will be won by brokenness and humility and sacrifice. It will be won when we identify with the children in our suffering rather than with the abortionist in his killing.Amen.
But the objections are still going to come fast and furious. If you believe that abortion is an American Holocaust, and if you would approve of taking out a Nazi executioner or Hitler himself in order to save lives, then how can you object to murdering a man who earned his living by killing fully formed babies day-in and day-out? Or to put it even more sharply: if you saw a thug about to kill a toddler, wouldn't it be justified to shoot him in order to stop the murder in progress?
Greg Koukl gives a fairly detailed answer here regarding the ethics and logic of the dilemma.
Update: Robert P. George weighs in:
Whoever murdered George Tiller has done a gravely wicked thing. The evil of this action is in no way diminished by the blood George Tiller had on his own hands. No private individual had the right to execute judgment against him. We are a nation of laws. Lawless violence breeds only more lawless violence. Rightly or wrongly, George Tilller was acquitted by a jury of his peers. "Vengeance is mine, says the Lord." For the sake of justice and right, the perpetrator of this evil deed must be prosecuted, convicted, and punished. By word and deed, let us teach that violence against abortionists is not the answer to the violence of abortion. Every human life is precious. George Tiller's life was precious. We do not teach the wrongness of taking human life by wrongfully taking a human life. Let our "weapons" in the fight to defend the lives of abortion's tiny victims, be chaste weapons of the spirit.