In a makeshift barracks about 40 miles south of Baghdad, Ahmed al-Zamili flipped through pictures on his mobile phone: an Islamic State fighter’s corpse hanging from a crude noose, a dead man on the ground clutching an AK-47 and a kneeling, blindfolded man uttering a confession.

Mr. Zamili says the men were captured when his militia of more than 650 Shiite fighters, known as Al Qara’a Regiment, drove Islamic State out of Jurf al-Sakher in late October. After briefly interrogating the enemy soldiers, Mr. Zamili ordered their executions, he says.

“We see them, we attack them, we get the weapons from them, we talk to them, we get their confessions, and then we kill them,” says Mr. Zamili, 35 years old, who ran five restaurants before forming Al Qara’a in June. “Of course, this is much better than the army strategy.” […]