I have no sympathy for Dr. Tiller. He was a mass killer of viable, innocent, unborn children. He was a man soaked with the blood of innocence.
Tiller is now in the hands of God and, very likely, in hell. After the judgement on the last day, if he is in hell now, he will spend eternity in the Lake of Fire. Still, as evil as Tiiler's work was, God did not give you or me the authority to execute anyone doing something that the state has declared lawful.
In the case of abortion, the 1973 Supreme Court declared evil to be good, and good to be evil, by making killing unborn children lawful throughout our land. It is a horrible evil that is sanctioned and protected by the United States government, and we know from Romans 13 that those raised to authority in our land are there because God has ordained them to rule over us. "Let every soul be in subjection to the higher powers: for there is no power but of God; and the powers that be are ordained of God." (Rom. 13:1)
When St. Paul wrote these words, Nero Caesar was the chief magistrate (dictator) of the Roman Empire. He was a very evil man. It was Nero who had St. Paul and St. Peter put to death. He had countless Christians murdered by the Roman State. Yet, Paul said that a beast like Nero was "ordained of God."
Abortion and infanticide were common in Roman Empire. The Earlier Church Fathers spoke out against the evil of baby killing, and Christians in the ancient church worked to stop the evil in godly ways. They did good. They did not stoop to murder.
In the Old Testament, the worshippers of Moloch sacrificed children to this blood thirsty, pagan god. The Prophets of Israel prophesied against their evil acts, but they did not hint that people could take the law in their own hands and kill the worshippers or priests of Moloch.
God does not give His people permission to do evil in the fight against evil. Who ever decides to be judge, jury and executioner, as in this case or in the case when defrocked, pastor Paul Hill murdered Dr. John Britton in Pensacola (1994), he or she declare by their actions, that they do not believe God or his Word. They think they know or can do better than God.
When government supports and defends evil, as ours does with abortion, we are not given authority from God to take the law into our own hands. To do so, shows that we deny God's sovereign rule over His creation. God will judge the evil in His time. Rome, promoted the persecution and murder of Christians for more that 200 years. Christians did not rebel or take up arms against Rome. They had no authority from the Lord to do so.
We are to face evil by doing good. God will take care of the Moloch worshippers, the Neros and the Tillers of this world in His own time. Men like Roeder and Paul Hill have done evil and the state has the authority to punish them for their crimes (Paul Hill was executed for murder in 2003).