Riders of the Purple Sage by Zane Grey

Venters looked quickly from the fallen rustlers to the canyon where the others had disappeared. He calculated on the time needed for running horses to return to the open, if their riders heard shots. He waited breathlessly. But the estimated time dragged by and no riders appeared. Venters began presently to believe that the rifle reports had not penetrated into the recesses of the canyon, and felt safe for the immediate present.

He hurried to the spot where the first rustler had been dragged by his horse. The man lay in deep grass, dead, jaw fallen, eyes protruding -- a sight that sickened Venters. The first man at whom he had ever aimed a weapon he had shot through the heart. With the clammy sweat oozing from every pore Venters dragged the rustler in among some boulders and covered him with slabs of rock. Then he smoothed out the crushed trail in grass and sage. The rustler's horse had stopped a quarter of a mile off and was grazing.