It’s an ancient story: An innocent idealist sets out to change the world and in the process becomes what he hates most. “He who fights monsters should see to it that he himself does not become a monster,” Friedrich Nietzsche advised. That’s probably the best spin one can put on Sidney Blumenthal, the longtime Hillary Rodham Clinton aide and confidant. But it would be just that — spin — given that Blumenthal was never an idealist, never mind an innocent. He has remained the same man he was in 1976, when he co-edited Government by Gunplay: Assassination Conspiracy Theories from...