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In the official teachings of the Roman Catholic Church, the distinction is also upheld.
“The deliberate murder of an innocent person is gravely contrary to the dignity of the human being, to the golden rule, and to the holiness of the Creator.”
The legitimate defense of persons and societies should not be considered as an exception to the prohibition of murdering the innocent.
Non-lethal force is always preferable, but if it is necessary, it must be seen in the context of the intended outcome, which is the preservation of innocent life.
“Injury or death to the aggressor is not the intended outcome, it is the unfortunate consequence of using necessary force to repel an imminent threat.”
Legitimate defense is not just the right, but also the grave duty of those responsible for the defense of the common good.
“Those who legitimately hold authority also have the right to use arms to repel aggressors against the civil community entrusted to their responsibility.”
All citizens and governments are obliged to pray and work toward the avoidance of war, but once all peace efforts have failed, the use of legitimate defense by a military force is licit.
“Such a decision is grave and therefore subject to rigorous considerations of moral legitimacy. The demands of a just war include:
- The damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain.
- All other means of putting an end to it must have been shown to be impractical or ineffective.
- There must be serious prospects of success.
- The use of arms must not produce evils and disorders graver than the evil to be eliminated.”
http://en.wikipedia.org/wiki/You_shall_not_murder. Quotations from The Catholic Catechism.