George put out his hand and grabbed Slim. "Wait a minute," he shouted. He cupped his hands around his mouth and yelled, "'Get 'im, Lennie!" ... Curley's fist was swinging when Lennie reached for it. The next minute Curley was flopping like a fish on a line, and his closed fist was lost in Lennie's big hand. George ran down the room. "Leggo of him, Lennie. Let go."
Why does George react like this when Slim offers to stop Curley from beating on Lennie ?