Traveling with the Dead, Barbara Hambly

Of course, Asher had been a spy. And when Jan van der Platz--sixteen and Asher's loyal shadow for weeks--had learned that Asher was not German but English and had confronted him in tears, Asher had shot him to protect his contacts in the town, the Kaffirs who slipped him information and would be horribly killed in retaliation, and the British troops in the field who would have been massacred by the commandos had he been forced to talk.