PRETORIA, South Africa — The world once cheered Oscar Pistorius, the “Blade Runner,” a double amputee whose gritty determination saw him become the first Olympian to race on twin carbon fiber blades.But on Valentine’s Day, it all went disturbingly wrong.South Africans awoke to the news that their country’s most famous sportsman had been charged with murder, accused of shooting and killing his gorgeous model girlfriend Reeva Steenkamp in the early hours of Thursday morning.The motive behind 29-year-old Steenkamp’s murder is still unclear.