Warning: This story contains descriptions of torture and physical violence. Some readers may find it distressing. A shopkeeper has told the BBC how Russian mercenaries fighting jihadists in Mali carried out the cold-blooded murder of two men in front of him and then threatened to chop off his fingers and kill him too. This is one of several similar testimonies collected by the BBC showing the tactics used by the Russian fighters as they waged a brutal counter-insurgency operation against