There are certainly times when it is just to mentally reserve information that another person does not have the right to know. For example, in the hypothesis offered, one can certainly say to the killer, “The old barn down the road,” and reserve the information about when the victim was at the old barn (e.g., last week).
In stressful situations, however, it can be difficult to methodically decide to reserve information. If one simply blurted out a lie in order to justly protect an innocent person, the situation would be similar to the person who takes a loaf of bread from a rich person in order to feed his starving family. In each case it may appear to be a sinful situation, but the underlying self-defense or defense of the innocent would mean that an action that was otherwise sinful was not sinful in this case.
Mental reservation and lying to protect the innocent are actions that can certainly be abused. If a person is doubtful as to whether his action was morally justified, he can ask a priest in the confessional. In addition to absolving sin, priests can also help us determine if a particular action was sinful.
