GÜIRIA, Venezuela — The host of a popular radio show, “The People’s Combat,” had always diligently praised Venezuela’s governing Socialist Party, even as millions sank into penury under its rule. But when acute gasoline shortages paralyzed his remote fishing town this summer, he strayed from the party line.

On his show, the host, lifelong Socialist José Carmelo Bislick, accused local party chiefs of siphoning fuel, leaving most people queuing for days outside empty gasoline stations.

Just weeks later, on Aug. 17, four masked, armed men burst into Mr. Bislick’s house and told him he had “run the red light,” before beating him in front of his family and hauling him away into the night. He was found dead with gunshot wounds hours later, dressed in his favorite Che Guevara T-shirt...