How can, under Catholic laws, the killing of another human being be morally justified?

St. Thomas Aquinas is regarded as one of the Doctors of the Catholic Church. This is because he established much of what the Church regards as Doctrine in reference to moral theology. One of these things was the question of a how a person can kill another in warfare, and be justified doing it. Thomas Aquinas based much of his theology on the earlier works of St. Augustine of Hippo, and also brought to light …

…death of another person must be sure that they have made morally right choices, and are only defending their own lives or the lives of other innocent people. The saying "the ends does not justify the means" applies more than ever in warfare and in the taking of human life, and therefore the taking of human life must be avoided if possible. Warfare and the taking of human life must be the last possible resort.