As they rushed around the corner they saw Shorty.

They saw Shorty first, because Shorty dominated the scene.

A gun in each hand, he made a terrible figure.

His eyes were blazing with the cold rage that had seized him at sight of Lawler, wounded--for Lawler was now leaning against the wall of the cabin, and his gun had dropped from his hand.
The unexpected appearance of Shorty startled Krell and Selden.

Surprise showed in their faces as they paused for an infinitesimal space and looked at him.
And then their guns roared.
Shorty, however, had anticipated them.