Ride Proud, Rebel! by Andre Norton, Science Fiction, Western, Historical

A man scuttles out of the brush -- and Drew only half sees the figure snapping a gunshot at him . . .

Feeling the sickening impact of the bullet in his middle, suddenly Drew cannot pull any air into his straining lungs. The reins fall from his hands -- but he clings to the saddle as the mule leaps braying ahead. Abruptly from beneath the mule's hoofs the ground gives way -- tumbling both of them into the icy stream Drew plunges into instant blackness, shutting out the terrible agony shaking him.

". . . dead," says someone above the boy.