Morality is not determined by the situation, but by God's laws. The choice is not of a lesser evil, but a greater good. Lying, in and of itself, remains wrong. Saying amputation is right in a given case does not justify the evil of mutilation in normal cases. Exemption v. Exception .
Exceptions undermine the moral law itself.
Exemptions mean the law violated still stands and must be respected. That law is binding in every other way. For example, God's Law demands respect for human life, and therefore says not to kill another human being. This is not an exception to the law demanding respect for human life. Yet it successfully recognizes the reality of true moral conflicts, and gives positive answers to these conflicts.