The vengeance that Iraq’s Shiite militias mete out as they fight the Islamic State group can be just as brutal as that of their sworn sectarian enemies.

In a grisly video recently posted online, a Shiite fighter shouts the name of a revered imam in victory as he poses beside decapitated bodies. Another militiaman sits nearby, grinning as he maims a corpse.

One bearded militiaman explains the bodies are those of fighters who “killed our comrades.” Another man shouts, “Our fighters were good guys. These are dogs.” […]