It is the picture which showed the true horror of the second-world war – a man blithely shooting Jewish prisoners and watching them fall into a mass grave. His identity has finally been revealed and it’s a stark warning about what can happen when we lose our humanity, writes Guy Walters A bespectacled and serious-looking young man is pictured standing, pistol in his right hand, pointing directly at the head of a Jewish man he is about to murder. He is photographed perched on the edge of a pit