We are assuming on this thread that it is an antiabortion activitist.
What if it isn't.
My first speculation was along those lines. Somebody's wife, girlfriend or daughter suffered serious emotional damage post-abortion. Non-random murder is usually over something personal.
It's less likely (but certainly quite possible) the shooter didn't have a personal attachment, and was acting "on principle." (scare quotes for obvious reasons, I'm sure honest readers understand the sort of person meant to be described by that phrase).