Evil has a reputation for resilience. And rightly so. Banishing it from Middle Earth alone took three very long Lord of the Rings movies. But equally deserving of this reputation is the concept of evil—in particular, a conception of evil that was on display in those very movies: the idea that behind all the world’s bad deeds lies a single, dark, cosmic force. No matter how many theologians reject this idea, no matter how incompatible it seems with modern science, it keeps coming back.

You would have thought St. Augustine rid the world of it a millennium and a half ago. He argued so powerfully against this notion of evil, and against the whole Manichaean theology containing it, that it disappeared from serious church discourse. Thereafter, evil was not a thing; it was just the absence of good, as darkness is the absence of light. But then came the Protestants, and some of them brought back the Manichaean view of a cosmic struggle between the forces of good and evil.

The philosopher Peter Singer, in his recent book The President of Good & Evil: The Ethics of George W. Bush, suggests that the president is an heir to this strand of Protestant thought. Certainly Bush is an example of how hard it is to kill notions of evil once and for all. On the eve of his presidency, in a postmodern, post-Cold War age, “evildoers” had become a word reserved for ironic use, with overtones of superhero kitsch. But after September 11, Bush used that word earnestly, vowed to “rid the world of evil,” and later declared Iran, Iraq, and North Korea part of an “axis of evil.”

So what’s wrong with that? Why do I get uncomfortable when he talks about evil? Because his idea of evil is dangerous and, in the current geopolitical environment, seductive.

Some conservatives dismiss liberal qualms...