St. Thomas taugjit that heretics should be put to death.

The Catholic Answers

He had in mind such men as had been Catholics, and who labored to destroy the faith of other Catholics after their own lapse from the Church. And even then he puts the question speculatively. And he was quite logical. He argued that one who unjustly takes his neighbor’s life by murder deserves death at the hands of the state. But he who destroys the faith of another robs him, not of his temporal life, but of his eternal life, which is far worse. The state, therefore, which is bound to safeguard the complete well-being of its citizens, would be justified in putting such a man to death, removing him permanently from among men to whom he can do so much damage. Speculatively, then, St. Thomas says that such a penalty would not be excessive. In practice he does not say that it should be done. Andeven if it were done, he writes that the Church whose mission is one of mercy must do all she can to win such a man from his sinful dispositions and destructive campaign, in order to save both his temporal and spiritual life if possible.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty