St. Paul the apostle taught that it is never lawful, even for the gravest reasons, to do evil that good may come of it (cf. Romans 3:8).
Most Rev. Charles Chaput, Archbishop of Denver and Most Rev. James D. Conley, Auxiliary Bishop of Denver gave the following clear statement:
"The violence at the heart of every abortion and the abortion industry itself will never be ended by counter-violence. The killing of George Tiller is an inexcusable crime that cuts against everything pro-life Americans, and especially religious believers, stand for.”
See more here: Catholic bishops condemn Tiller slaying