It’s painful to see people straining to justify St. Thomas Moore’s actions, as if we have to make excuses for them.
In reality, the execution of public and notorious heretics is no more sinful than the execution of a mass murderer. In fact, it’s much more justifiable. Murderers only kill bodies, while heretics damn souls - perhaps the souls of millions.
Obviously, punishments for heresy would be unworkable in our modern, pagan society, but they were positively laudable when they were enacted in previous times. Many souls were spared by these actions.
Here’s St. Thomas Aquinas on the subject, responding to objections which say that heretics must be tolerated. He puts it better than I could:
"
Article 3: Whether heretics ought to be tolerated?
…
*
On the contrary, The Apostle says (Titus 3:10-11): “A man that is a heretic, after the first and second admonition, avoid: knowling that he, that is such an one, is subverted.” *
I answer that, With regard to heretics two points must be observed: one, on their own side; the other, on the side of the Church. On their own side there is the sin, whereby they deserve not only to be separated from the Church by excommunication, but also to be severed from the world by death. **For it is a much graver matter to corrupt the faith which quickens the soul, than to forge money, which supports temporal life. Wherefore if forgers of money and other evil-doers are forthwith condemned to death by the secular authority, much more reason is there for heretics, as soon as they are convicted of heresy, to be not only excommunicated but even put to death. **
On the part of the Church, however, there is mercy which looks to the conversion of the wanderer, wherefore she condemns not at once, but “after the first and second admonition,” as the Apostle directs: after that, if he is yet stubborn, the Church no longer hoping for his conversion, looks to the salvation of others, by excommunicating him and separating him from the Church, and furthermore delivers him to the secular tribunal to be exterminated thereby from the world by death. For Jerome commenting on Galatians 5:9, “A little leaven,” says: “Cut off the decayed flesh, expel the mangy sheep from the fold, lest the whole house, the whole paste, the whole body, the whole flock, burn, perish, rot, die. Arius was but one spark in Alexandria,
but as that spark was not at once put out, the whole earth was laid waste by its flame.”
**
newadvent.org/summa/3011.htm#article3
