In the book Of Mice and Men, who is the only one who realizes what really happened between Lennie and George at the end of the story?

After George shoots Lennie, he leads the others to believe that he and Lennie had a struggle that ended in Lennie's death.  The rest of the men accept George's story at face value and do not give Lennie's killing a second thought, except for Slim.  Throughout the story, Slim is the character to whom the other workers turn for advice.  He is the voice of reason and rationality.  Slim listens to the others and helps them to solve their problems. The rest of the men were ready to kill Lennie themselves as retribution for Curley's wife's death.  Prior to going to find Lennie,  George approaches Slim to determine what would be best for Lennie.  Would it be ok to just bring him in to the authorities?  Slim suggests that having him arrested would not be the best solution for Lennie's well-being.  Therefore, when he finds George next to Lennie's dead body, he understands exactly what had transpired.