After barely escaping from a ranch up in Weed, Lennie end George get to work on a ranch south of Soledad. Everything works out just fine, until Lennie has a bad accident with the wife to Curley, the boss's son. As I said, Lennie is not so normal, so can't control his strength right. Without meaning it, Lennie breaks the woman's neck. He gets frightened and run off, to hide in the brushes. When the men at the Ranch discover this crime, they understand that Lennie is the killer. They take their dogs and shotguns, and go out to find and kill Lennie. George waits until the men have left, cause he knows where Lennie is. He now has three choices: He could find Lennie and run away with him, he could let the other men kill Lennie, or he could kill Lennie himself. He chose the last option.
I think it was right of George to kill Lennie himself. If they had decided to run away together, they would probably been caught. And if them both got caught, they would probably both get shot. The men would think that George had helped Lennie killing Curley's wife. And George obviously didn't want to die.
George didn't have heart to let the other men kill Lennie. He knew that Lennie wouldn't make it out alive of this mess,