Redcoat Renegades by Douglas Hirt
Kit rides into the shadow of the Mission Mountains, the wildest of the wild country, where the Flatheads and Blackfeet clash and have for eons, but there's peace...until a renegade former redcoat stirs a caldron of hate and blood...and if Kit's not careful, he'll spend eternity in those shadows!
When Kit Carson arrives in the rich fur-trapping country of the Flatheads, he finds unexpected enemies waiting for him. A renegade British fur agent has been stirring the local tribes into a war of vengeance against the Americans, including their new captive--Kit. Suddenly, it falls to Kit to stop the war before it starts or become the first casualty. .