The Latin American Herald Tribune reports Fidel Castro basically sent Che Guevara to his death in Bolivia because Che had become an irritant. The article doesn't say so, but I suspect Fidel was jealous of Che's charisma.
It's Che's picture on the t-shirts, not Fidel's; Che was the rock star. Ironically, all Fidel accomplished was martyrdom for Che, his legend lives on, unsullied by the inconvenient details of his brutality.
