Adolf Eichmann infamously cited Kantian ethics as justifying his banal servility towards the higher ranks of the Nazi regime; Himmler attempted to cultivate in the SS (the special murder corps of the regime, so to speak) an attitude of clinical detachment. Whether the doctors who worked in Auschwitz, say, were so detached in their death clinics, I don't know (I've read little about the Doctor's Trial), but it's conceivable that these people thought of themselves as acting on some sort of "reasonable principle of the matter."
One would recommend Hannah Arendt's The Origins of Totalitarianism for analysis of the "murderous logicality," the (supposedly) impeccable, but mere, consistency of both the Nazi and Soviet regimes' destructiveness with the premises of their worldviews:
Once their claim to total validity is taken literally they become the nuclei of logical systems in which, as in the systems of paranoiacs, everything follows comprehensibly and even compulsorily once the first premise is accepted. The insanity of such systems lies not only in their first premise but in the very logicality with which they are constructed. The curious logicality of all isms, their simple-minded trust in the salvation value of stubborn devotion without regard for specific, varying factors, already harbors the first germs of totalitarian contempt for reality and factuality. [1962 ed., 457-8]
The Origins is not a perfect book, and is rhapsodical even, but she cites her sources fairly well, and writes with the poetry of her people's suffering shining through like a cry spanning both Heaven and Hell.
But now then from Socrates (in the Meno) to the scholastics,S to Immanuel Kant indeed, and later Donald Davidson, it has been thought that there is something intrinsically deficiently rational, or even counter-principled, about incorrect practical/unethical thought. This is somewhat reflected in the debate over generalism and particularism in ethics (see also the less neutral article on just moral particularism in particular). With respect to Kant again, he denies the human possibility of a diabolical will, finding the worst human evil not in an outright rejection of reason as the faculty of principles but in an inversion of two deep principles' ordering over and/or against each other.
SAs they have put the question to themselves:
And in the first place what was the nature of the sin of the rebel angels? In any case this was a point presenting considerable difficulty, especially for theologians, who had formed a high estimate of the powers and possibilities of angelic knowledge, a subject which had a peculiar attraction for many of the great masters of scholastic speculation. For if sin be, as it surely is, the height of folly, the choice of darkness for light, of evil for good, it would seem that it can only be accounted for by some ignorance, or inadvertence, or weakness, or the influence of some overmastering passion. But most of these explanations seem to be precluded by the powers and perfections of the angelic nature. The weakness of the flesh, which accounts for such a mass of human wickedness, was altogether absent from the angels. There could be no place for carnal sin without the corpus delicti. And even some sins that are purely spiritual or intellectual seem to present an almost insuperable difficulty in the case of the angels.
- Reasons for Action: Internal vs. External
- Reasons for Action: Justification, Motivation, Explanation
- Structural Rationality
- Radical Evil: Four Conceptions
- Christine Korsgaard, The Sources of Normativity