what does slim make curly agree to ? why does he make curley do this? what does this tell you about slims character?

Before Curley goes to the hospital, he agrees to pretend that he has caught his hand in a machine. Slim threatens to tell others what really happened if Curley does not agree to this.  Lennie is afraid that he has done something bad, but George reassures him that he hasn't as the chapter closes. Slim likes George and Lennie and does not want to see them fired. Slim is also a fair and just man. He knew that Curley deserved what happened to him.