Dear Diego:

You are correct. We cannot intend on killing someone on purpose, but in the course of protecting oneself and one's family killing the person may be required. The intent here is to stop the attack, not to kill. The death of the intruder is an unintended side-effect.

The concept of proportional force applies here. Even a police officer cannot use disproportionate force. If we apply deadly force when a lessor force is appropriate, then we commit a crime. Exactly where that line is drawn can be hard to determine. A small woman, for example, may be in fear of her life when her attacker is strong man, even if unharmed, thereby perhaps allowing deadly force to be appropriate for her.

But, if one is in fear of his life or the life of others and the only way to stop the intruder is to shoot them, then no crime is committed and no sin.

However, if your friend shoots someone in the back who is running away, since the threat to him is gone and there is no justified reason to use deadly force, he will likely be be charged with a unjustified homicide, perhaps even 2nd Degree murder, which usually gets a 25 year - life sentence, depending on the state.

The Catechism states:

2263 The legitimate defense of persons and societies is not an exception to the prohibition against the murder of the innocent that constitutes intentional killing. "The act of self-defense can have a double effect: the preservation of one's own life; and the killing of the aggressor. . . . The one is intended, the other is not."

2264 Love toward oneself remains a fundamental principle of morality. Therefore it is legitimate to insist on respect for one's own right to life. Someone who defends his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow:

If a man in self-defense uses more than necessary violence, it will be unlawful: whereas if he repels force with moderation, his defense will be lawful. . . . Nor is it necessary for salvation that a man omit the act of moderate self-defense to avoid killing the other man, since one is bound to take more care of one's own life than of another's.

2265 Legitimate defense can be not only a right but a grave duty for one who is responsible for the lives of others. The defense of the common good requires that an unjust aggressor be rendered unable to cause harm. For this reason, those who legitimately hold authority also have the right to use arms to repel aggressors against the civil community entrusted to their responsibility.

God Bless,
Bro. Ignatius Mary