So, is killing an abortion doctor a selfish act? What do I gain by killing an abortion doctor? I don't necessary gain anything... Maybe honor, pride, I'm not sure? Probably not even that.
You gain a sense of justice in knowing that you have saved innocent lives. Ideally you could just lock these people in jail, but that's not going to happen unless you can convince the majority of Americans (who are complicit in the killings and who thus might also deserve to die) to make it illegal.
Based on your premise that abortion is murder, killing an abortion doctor or an abortion advocate is no different in principle than jailing or executing a mass murderer.
What do abortions doctors get when they (legally) kill these babies? Money... Selfish act...
They aren't doing it for money! The amount of harassment that they suffer and the threat of being killed or having family members killed is cost prohibitive.
They are doing it because they are atheist or secular, they don't believe that a person lives inside of a fetus that barely has a brain, and they feel they are helping women (and the would-be fathers) take control of their lives, preventing 18+ years of unnecessary poverty and suffering.
I suspect that the abortion doctors almost religiously believe that what they are doing is good and righteous just as much as abortion opponents believe that it is sinful.
What we have here is a clash of two philosophies:
One philosophy believes that we exist on this earth and that the good is to live your life and enjoy it. In this view, a God doesn't exist or if it does, it doesn't care about us and we are insignificant to it. Also, in this view, cell masses are not people and fetuses that barely have brains cannot possibly be people. So, there's nothing wrong with ending a pregnancy because no people are being harmed and potentiality is not actuality. Some Christians who are really Christian in name only fall into this camp. (They claim to believe in the other philosophy but in practice, based on how they live, they really believe in this philosophy.)
The other philosophy believes that a magic God-being that transcends all of the physical laws we know, that transcends reason and logic, exists. It's also implied that the good is to worship this God-being and follow its dictates, sacrificing things that seem good for you on this earth, perhaps in the hopes of a reward in the after-life. This God commands us not to kill these cell masses and fetuses. According to one version of this belief, the God-being "breathes" a "soul" into the cell mass at the time of conception. The very formation of the cell mass itself is thus a miracle and ending it--opposing the will of God--constitutes a grave sin.
That's what's going on here--you have a clash between two diametrically opposed philosophies. One of them is a "this worldly" philosophy and the other is mystical. The difference in these belief systems goes to the very core of everyone's thoughts and beliefs. It goes right down to basic metaphysics and what you believe to be the very nature of reality.
The implementation of the first philosophy led to an enlightenment and the development of modern technology and religious and social freedom. The implementation of the second philosophy led to the Dark Ages and religious dictatorship.