Soldiers in Ukraine are killing Russians as they surrender.

Hours after a battle in eastern Ukraine in August, a wounded and unarmed Russian soldier crawled through a nearly destroyed trench, seeking help from his captors, a unit of international volunteers led by an American.

Caspar Grosse, a German medic in that unit, said he saw the soldier plead for medical attention in a mix of broken English and Russian. It was dusk. A team member looked for bandages.

That is when, Mr. Grosse said, a fellow soldier hobbled over and fired his weapon into the Russian soldier’s torso. He slumped, still breathing. Another soldier fired — “just shot him in the head,” Mr. Grosse recalled in an interview.