For what it’s worth, my position is that, given a particular moral judgment, a particular course of action is necessary.
For example, suppose tomorrow I am presented with persuasive evidence that cars are sentient beings on a level with humans who wish to sit in driveways and meditate on the nature of cement; that movement is agonizing for them; that oil companies know this and devised a nefarious torture called gasoline that, when poured into a car, so badly hurts it that it’s willing to move around to avoid further gasoline torture.
If I were convinced of that, I’d be morally obligated to stop driving; what’s more, someone exposed to this evidence as I was who continued driving would be terribly evil. Someone who rescued cars from such a fate by killing people who continued operating them (slavedrivers, we’ll call them) wouldn’t be such a bad person.
Of course, I have no such evidence. Absent such evidence, if I started killing drivers now, I’d be a terrible person.
The scenario is ridiculous, of course–even more ridiculous than the scenario under which anti-abortion murderers operate. And it’s that absurdity that separates anti-abortion murderers from anti-slavery murderers.
“But how can you know that your moral judgment is the right one?” I hear you ask. Answer: I can’t. But it’s the one I have, and I have to balance that uncertainty against the effects of doing nothing. Sometimes the balance comes out in favor of doing nothing. Other times it doesn’t.
Chattel slavery is wayyyyy on the far end of the extreme, stopping just short of the Holocaust. It’s pretty easy to know that freeing people from chattel slavery is a good thing, even if violence against slavers is required in the process.