Killing Abortion Doctors

In a sparkling post that ignited a flurry, Rob argued among other things that killing 12 year olds is different from killing fetuses, and that if the former took place we would not stand idly by. As I recall, only one post addressed the comparison and argued to the contrary that killing a fetus was more serious because of its relative vulnerability. Assuming it would be morally permissible to kill a doctor to prevent the killing of a 12 year old (that is, assuming you do not take a pacifist position), is it morally permissible to kill abortion doctors? Does the legality of abortion make a difference? Why should it if you think abortion law is immoral and unjust? For a thoughtful discussion of many of the issues raised by a "pro choice" proponent who tries to think within the "pro life" framework, see Sherry Colb http://writ.news.findlaw.com/colb/20080109.html.  I am interested in why those who are strongly in the "pro life" camp think that such killing would be immoral (to the extent they do) especially if they have something to add to the arguments made by Colb.