I call troll, this to bait us into giving our stance on those who kill abortionist.
I asked the question and I reveal my identity now to show that I am not a troll. I could ask if it were lawful to kill someone like Stalin or Mao to prevent them from killing many people. Your example of killing an abortionist works too as the same principles apply.
The last few Popes with their modernism and error have caused the destruction of many souls, which is a graver crime than killing people's bodies.
Who would seriously suggest killing those Popes as a solution?
I think, therefore, when you kill a Stalin or a Mao they have to be a tyrant who is ready to fall and where they have little or no supporting power-base. Usually people like that are living on borrowed time anyway. Otherwise you don't really solve any problem.
Like a bad Pope, a bad tyrant can be replaced with an equally bad or worse tyrant. In fact, your killing them only makes that more likely.
The 'best' moral justification I can see is the one for killing abortionists. You can be reasonably certain that they will kill again tomorrow, and the day after, and unlike tyrants they are not THAT easy to replace. Most doctors would not want to do abortions, (it is a very unpleasant and grissly business). So if 100 or so abortionists were shot dead worldwide, it would have an impact on the number of abortions that could be done. One abortionist does A LOT of abortions. Some of those women, if abortions were difficult to access, would NO DOUBT reconsider their options and keep the baby. I would hazard a guess that the number of babies saved would be an order of magnitude greater than the abortionists killed. (Not that that alone justifies it)
But prayfully picketing the clinics is more effective I think (and harder in someways). Violence is exciting and very attractive, (instant gratification) but not usually very effective in terms of achieving the outcome you want.