Many modern historians are reluctant even to deploy terms like "good" and "evil", though these abounded on all sides during the war, so one is obliged to engage with them in the interests of verisimilitude. We should not be so ahistorical as to reject this terminology just because it is camouflaged in a theological fog. In the real world, judges routinely use the term evil so as to distinguish between, say, a married person at the end of their tether who stabs their spouse in the heat of the moment and a person who cold-bloodedly decides to torture and kill a succession of women or children without the excuse of some major psychotic compulsion. 

That seems to apply to the German criminal conspiracy to murder Europe's Jews, a deed accompanied by enormous detailed cruelty, despite efforts to make the operation seem clinical and industrial. It would also apply to the gratuitous sadism the Japanese (and their allies such as the Koreans) brought to the treatment of internees and PoWs, which was strikingly at variance with the civilised way in which the Japanese had behaved towards Russian captives in 1904-05 — that is, before they consciously decided to reject Western humanitarian values. 

Does this have any relevance to Hennessey's contemporary war stories, which are generically related more to the separate historical stream of colonial counter-insurgency warfare? Certainly yes, on the level of how individual troops conduct themselves in wars where the distinction between combatants and civilians is blurred. And yes, too, in terms of how our government needs to make the case again and again for what young men and women are fighting for in Afghanistan, a war the majority of British people seems to think is a lost cause.