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.

Mr. Grosse said he was so upset by the episode that he confronted his commander. He said he spoke to The New York Times after what he regarded as unwarranted killings continued. It is highly unusual for a soldier to speak publicly about battlefield conduct, particularly involving men whom he still considers friends.

The shooting of the unarmed, wounded Russian soldier is one of several killings that have unsettled the Chosen Company, one of the best-known units of international troops fighting on behalf of Ukraine.

Mr. Grosse’s witness recollection is the only available evidence of the trench killing. But his accounts of other episodes are bolstered by his contemporaneous notes, video footage and text messages exchanged by members of the unit and reviewed by The Times.