He had his hand at Sam's throat, and was trying to choke him.

Sam saw that one great effort was necessary, and with a heave of his whole body, threw the other beneath him, and struck downwards, three quick blows, with the whole strength of his ponderous fist, on the face of the man, as he lay beneath him.

The hold on his throat loosened, and seeing that they had rolled within reach of his sword, in a moment he had clutched it, and drawing back his elbow, prepared to plunge it in his adversary's chest.
But he hesitated.

He could not do it.

Maddened as he was with fighting, the sight of that bloody face, bruised beyond recognition by his terrible blows, and the wild fierce eyes, full of rage and terror, looking into his own, stayed his hand, and while he paused the man spoke, thick and indistinctly, for his jaw was broken.
"If you will spare me," he said, "I will be King's evidence." "Then turn on your face," said Sam; "and I will tie you up." And as he spoke a trooper ran up, and secured the prisoner, who appealed to Sam for his handkerchief.