Ramesh Ponnuru rightly responds that "Not a single step of Saletan’s logic is valid." He concentrates on the first invalid move:
Hypothetical scenarios can always be spun out to attempt to justify the unjustifiable. But to even begin to construct a bridge from the humanity of unborn life to the justifiability of shooting Tiller [1] he would have to be in the act of committing an abortion, [2] the shooter would have to know to a moral certainty that no one else would perform the abortion, [3] the goal would have to be to disable rather than kill him, and [4] it would have to be possible for a pro-life regime to survive without the rule of law. These are impossible conditions.Read the whole thing.
[Numbered brackets in Ponnuru's quote were added by me.]