Killing abortion doctors is morally wrong, but may be legally right.

Killing an abortion doctor is morally wrong at least for the same reason that futile resistance to unjust aggression is morally wrong: Without the hope of success, resistance merely increases the harm. Recourse to arms in the case of abortion is futile for two reasons: (1) It only delays rather than stops the particular abortion, because the mother is almost certain simply to reschedule her abortion with another doctor. (I say "almost certain" because other abortionists are easily available today and because an act of force -- as opposed to a peaceful sit-in, together with a willingness to suffer arrest for the sake of her child-- will appear merely violent and thus will not be likely to change the heart of any pregnant woman.) (2)  The killing an abortionist will frighten many Americans, especially since it will be portrayed simply as violence rather than as rescue, and so will close their minds to pro-life arguments. But the only hope of the pro-life side is in the truth of its arguments, since it lacks almost all power. It would be exceedingly immoral so to damage the long-range chances of stopping abortion.

Nevertheless, if some pro-life person did kill an abortionist, although he would in my view have done a very immoral act, he should be defended in court. First of all, an unjust law (like Roe v. Wade, which violates equal protection) is not valid law, according the Evangelium Vitae. But even if Roe were valid, that case only forbids state action to stop abortion. But I assume that the pro-lifer here does not act under cover of state law. So I think he or she has a necessity defense, under Model Penal Code standards, just as (as I recall) a woman in Michigan was recently held to have a necessity defense when she used force to protect her unborn child. A child does not have to count as a constitutional person for the necessity defense to be used. 

The necessity defense is arguably concerned more with the immediate consequences of an act, as opposed to the longer term consequences which (I have argued above) make such an act immoral. (Indeed, in light of our long tradition of wide-open prosecutorial and jury discretion, it seems to me possible that our appellate courts would refrain from imposing a duty to convict. That is, the necessity defense seems to me not only technically valid here; it might even stand a chance of being successful, with open-minded people in the jury box and on the bench.)