This article by Ross Douthat is important for all pro-life people to read. He does two major things.
1) He shows why pro-life rhetoric should not be blamed for vigilantes (especially mad ones) like Robert Dear:
Pretty much all passionate, justice-seeking movements risk attracting a violent fringe, and while activists in those movements are obliged to condemn and marginalize that fringe, their larger case and public rhetoric can’t be held hostage to the fear that someone, somewhere, might commit violence in their name. Otherwise the Black Lives Matter movement would be obliged to change its name to Black Lives Matter, But Not In The Kind of Immediately Life-and-Death Way that Might Justify Violence Against Police Officers every time a protestor throws a brick or a cop gets assaulted. (And, to be clear, that kind of change would be really, really stupid.)
2) He probes the issue of whether or not vigilante violence is justified in the defense of the unborn:
So let’s dig in to the question. Given anti-abortion premises, why is it not obviously reasonable to take up arms against abortion providers? Why isn’t the pro-lifer who shoots an abortionist just like a man or woman who uses deadly force against a would-be child murderer—a vigilante, yes, but also a heroic one?
I found his entire discussion very helpful.