If you believe abortion is murder ( I do not except late term) than the killing of the doctor is an act that can be rationalized. We kill terrorists to stop them from killing others. Yet the rationale is not the same because the Dr was performing a legal activity that will be performed by someone else.
It is okay to oppose abortion, but one can not oppose immorality with terrorism. That being said the vast majority of the pro life people are peaceful.
Abortion is a matter where it is a personal belief. Even though I am pro choice I respect the sincere views of the pro lifers.