While the Catholic Church has taught abortion is murder since 1869 (and it has been a sin nearly since the start of the Church), the fact remains that abortion is legal, and (AFAIK) almost always done at the request of, and with the consent of, the mother. While violence may be justified to protect one's life or the life of another, the use of violence in the case where an act is legally sanctioned by the legitimate public authority is not justified, nor is it sanctioned by the Church.
A similar case may be made for euthanasia, assisted suicide, or capital punishment. While an obligation exists to preserve the dignity of life, that obligation does not get fulfilled through violence. In the case where the government has deemed acts contrary to the dignity of life legal, the obligation must be fulfilled through conversation, discussion, and changing the minds of others.