It’s well known that left-wing Soviet leader Joseph Stalin was a psychopath, a dark status that led to his exterminating 20 million human beings. Yet the people who help vault such tyrants to power — sometimes pejoratively called “useful idiots” — are well-meaning but naive sorts, right? Perhaps not always. In fact, a new study informs, Stalin has much psychopathic (and narcissistic) company among leftists, including among today’s variety. Moreover, the researchers found that authoritarian-minded left-wingers often don’t care a whit about the altruism and “social justice” they trumpet, but instead “use political activism to endorse or exercise violence against...