309. THE FIFTH COMMANDMENT
You are not to commit murder.
This commandment, 'You are not to commit murder', means in the natural sense that it is forbidden to kill a human being or inflict upon him any wound that might prove fatal; also to mutilate his body. It also means not doing any drastic injury to his name or reputation, since to many people their reputation is as precious as their life. In a wider natural sense murder includes feelings of enmity, hatred and revenge which are murderous in intent. For these have the idea of murder lurking in them, like fire in wood beneath ashes; and the fire of hell is precisely this. This is why we talk of burning with hatred and being hot for vengeance. These are murders in intention, but not in deed. If fear of the law, of retaliation and being punished in the same way were removed, such people would break out into actions, especially if the intention contains an element of deceit or savagery. These words of the Lord prove that hatred is a kind of murder:
You have heard that it was said by the men of old, You are not to commit murder, and anyone who kills will be subject to judgment. But I tell you that anyone who is angry with his brother unjustifiably will be subject 1 to the hell of fire, Matthew 5:21-22.
The reason is that everything intentional is an act of the will, and thus is essentially a deed.
1. The quotation is abbreviated at this point, possibly by error.