Lying is intrinsically evil, and therefore always immoral. However, not all lies are grave matter. The assertion that an act is intrinsically evil does not tell us its moral weight. Some lies are objectively venial and others are objectively mortal.
There are three fonts of morality, three things that make an act moral or immoral:
- moral object
All intrinsically evil acts have an evil moral object – independent of intention or circumstances. The moral object is not your intention or intended end; it is not the purpose of the act (the reason for performing the act). That is the first font.
The moral object is the end, in terms of morality, toward which the knowingly chosen act is ordered, by its very nature. When that end is deprived of a good required by the love of God and neighbor, the object is evil and the act is intrinsically evil.
Circumstances are judged by the totality of the foreseeable consequences for all persons affected by the act. It is always wrong to act when we reasonably anticipate that our choice to act will do more harm than good.
Lying is intrinsically evil regardless of intention or circumstances. The second font in the act of lying is the direct and voluntary deprivation of truth from an assertion. When you deliberately knowingly assert a falsehood or deny a truth, you lie. The intention to deceive is the most common intention, but it does not determine the moral object. Lying is wrong for any intended end, in any circumstance.
Murder, defined by the second font, is the direct and voluntary killing of an innocent human being. Killing a human being who is attempting to kill you, or who is a soldier unjustly attacking your nation is not murder, because the person is not innocent.
Murder is immoral regardless of whether it is intended as an end, or as a means to an end. The intended end and the circumstances do not change the moral object. Murder is wrong for any intended end, in any circumstance.