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 many theories, such as the principle of Double Effect and situational ethics. In this essay, I will be discussing the theory of Double Effect, and how it justifies killing another person in war or otherwise, the effect of situational ethics on the taking of human life and also the catholic churches' teachings on a 'Just War.'

Saint Augustine of Hippo was the first person to write down a set of ideals in the fifth century on an ideal that was profoundly difficult: how to reconcile traditional Christian teaching against the use of violence with the need to defend the Roman Empire - who had been Christian for more than a century by this time - from the invading Vandals.

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St. Thomas Aquinas (c. 1225-1274), the eponym of T...

Thomas Aquinas depicted in stained glass

The solution he reached - a justification of war under certain prescribed circumstances, yet with genuine limits on the harm that could be done even in a justified war - is regarded as the beginning of the just war doctrine in Christian teaching. Augustine came up with two terms; one to define what is just to do in wars, which is jus in bello and the other pertaining to circumstances that are just cause for a war is called jus ad bellum.

Augustine first formulated the theory for killing another person that did not directly involve defence of the individual directly. This is because "Self-defence...