If the third tri-mester fetus cannot be called an innocent life, who can?
I did ask the moral question, do we not have an obligation to prevent those murders?
As to the question of what obligation exists to prevent murder via abortion, that obligation stands in conflict with other requirements, such as those to be responsible to your family, to obey the civil authorities, and to live peaceably in society, among others.
The first point is that no one, not even those in authority, can absolutely abolish abortion. Were abortion to be outlawed tomorrow entirely in all 50 states, abortions will continue to occur. There were plenty of underground abortionists pre-Roe v. Wade; for example, Frank Sinatra's mother was an abortionist in New Jersey during the 1930s. All 50 states have laws against murder, rape, and robbery, yet these crimes happen. Until the Second Coming, these crimes will continue to occur.
The second point is that when you commit an act of murder to kill an abortionist, you will likely go to jail for a long time and in some states, you will be sent to the execution chamber. A person who does so will be unable to support his parents, his spouse, or his children. He may be in violation of the Fifth Commandment, if his parents need support, and of 1 Timothy 5:8 which tells us, "But if anyone does not provide for his own, and especially for those of his household, he has denied the faith, and is worse than an unbeliever." (ESV)
The third point is that punishment of wrongdoing is the role of the civil magistrate, not a self-appointed vigilante. I have dwelt on this point at length previously. The fact that government has failed to protect the lives of the unborn does not justify a violation of the Sixth Commandment.
The fourth point is that we are to live peaceably in society. Romans 12:18 states, "If possible, so far as it depends on you, be at peace with all men." (NASB) As Matthew Henry states in his commentary: "...if our enemies are not melted by persevering kindness, we are not to seek vengeance; they will be consumed by the fiery wrath of that God to whom vengeance belongeth." Committing an act of violence outside of the realm of self-protection or of Biblical duties to protect family and country stands outside this command.
In summary, while we are obliged to help our neighbor, such obligation does not supercede other Biblical obligations, such as obedience to authority, duty to family, and a life at peace.