The implication I got is the men Billy found himself among men who didn't respect him (saw him as a walking wallet) and murdered him after he was no longer usef

The implication I got is the men Billy found himself among men who didn't respect him (saw him as a walking wallet) and murdered him after he was no longer useful to them (they plausibly considered him a liability in combat since he didn't fit the mold of a soldier.) However I don't know if there was combat reported in the vicinity of Dibrivka during that particular time period. It's possible he was killed in combat. > "Billy was nearly the opposite—5-foot-7, 160 pounds, his features rounded by a diet of snacks and fast food. His scruffy brown beard offset a receding hairline; rectangular metal-frame glasses gave him a bookish look." > "Mr. Polynkov sized up his new recruit as a traveler, not a fighter. In his experience, soldiers of fortune didn’t travel with wheeled oversize suitcases. He tried to dissuade Billy from going to Rostov [...]" > "Men at the camp forced Billy to pay their liquor tabs. “He was naive,” Mr. Victorov said. Billy’s credit cards registered more than $2,700 in fraudulent purchases and cash advances in Russia before he disappeared."