This is a really excellent technique in a lot of contexts.

I offer a word of caution about actually using it with theists, even those less Biblically literate than Yvain's friend: the catch-all excuse that many (not all) theists make for Biblical atrocities is precisely that they were commanded by God, and thus on some version of Divine Command Theory are rendered okay - not that the atrocities are in some observable way actually less bad than those committed by other groups or religions.