The distinction between the intended, and the merely foreseen, effects of a voluntary action is indeed absolutely essential to Christian ethics. For Christianity forbids a number of things as being bad in themselves. But if I am answerable for the foreseen consequences of an action or refusal, as much as for the action itself, then these prohibitions will break down. If someone innocent will die unless I do a wicked thing, then on this view I am his murderer in refusing: so all that is left to me is to weigh up evils. Here the theologian steps in with the principle of double effect and says: “No, you are no murderer, if the man’s death was neither your aim nor your chosen means, and if you had to act in the way that led to it or else do something absolutely forbidden.” Without understanding of this principle, anything can be—and is wont to be—justified, and the Christian teaching that in no circumstances may one commit murder, adultery, apostasy (to give a few examples) goes by the board. These absolute prohibitions of Christianity by no means exhaust its ethic; there is a large area where what is just is determined partly by a prudent weighing up of consequences. But the prohibitions are bedrock, and without them the Christian ethic goes to pieces. Hence the necessity of the notion of double effect.
(G. E. M. Anscombe, “War and Murder,” chap. 3 in Absolutism and Its Consequentialist Critics, ed. Joram Graf Haber [Lanham, MD: Rowman & Littlefield Publishers, 1994], 29-40, at 37 [essay first published in 1961])