He was now certain the men were what he had taken them to be, and he grinned felinely as he squirmed around until he got into a position from which he could see them.

But when he did get into position the men had vanished.
However, Sanderson was not misled.

He knew they had secreted themselves behind some of the rocks in the vicinity, no doubt to wait a reasonable time before endeavoring to discover whether the bullet had accomplished its sinister object.
Sanderson's grin grew broader.

He had the men at a disadvantage.
Their horses, he had observed before calling to them, were in a little depression at the right--and entirely out of reach of the men.
To get to them they would have to expose themselves on an open stretch between the spot where the horses were concealed and the hill on which they were secreted, and on the open stretch they would be fair targets for Sanderson.
The men had brought Sanderson into the fight, and he no longer had any scruples.

He was grimly enjoying himself, and he laid for an hour, flat on his stomach behind the rock, his rifle muzzle projecting between two medium-sized stones near the base of the large rock, his eye trained along the barrel, watching the crest of the hill on which the men were concealed.
The first man was dead.