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  he would have to be in the act of committing an abortion,  the shooter would have to know to a moral certainty that no one else would perform the abortion,  the goal would have to be to disable rather than kill him, and  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.]