Roeder did know that the man he killed wasn't the only doctor in the country who provides abortions, didn't he? Abortions are still legal, and women are still having them.
The way Roeder and terrorists like him see it, they're saving the world one abortion doctor at a time.
Tiller was especially targeted because he was one of a very few (like, count them on one or two hands) American doctors who perform late-term abortions. The Anti-choice brigade and their megaphones like Bill O'Reilly would have their followers believe that it's like any other abortion. "Hey, I'm 8 3/4 months pregnant but I suddenly decided I don't want the kid."
In reality, they're rare and generally performed only in extreme cases of medical risk to fetus and/or mother.