What advice does Slim give George about finding Lennie instead of killing him?

Slim's an authority figure at the ranch, someone the other men instinctively turn to for advice. So it's not surprising that George should pick his brains over what to do about the Lennie problem. But what Slim has to tell George isn't what he wants to hear. George wants to do everything he can to protect Lennie, but Slim knows that's impossible. He's painfully aware of the fate that awaits Lennie if George should turn him in to the authorities; he'll be caged like a wild animal, and there's no way that someone of Lennie's limited intellectual capacity will be able to handle such an ordeal.

Slim doesn't come right out and tell George that he needs to kill Lennie, of course; that's not his way. Instead, he simply lays it on the line, pointing out all the difficulties involved in protecting Lennie and allowing George to draw his own conclusions.

What advice does Slim give George about finding Lennie instead of killing him?

Towards the end of Of Mice and Men, Slim and George walk into the barn and discover Curley's wife is dead. Curley then enters the barn and says he's going to shoot Lennie for killing his wife. Slim quietly tells George that Lennie must have been responsible for her death even though it was an accident. George then tells Slim that Lennie would have traveled south since they came from the north. George asks Slim if they could find Lennie and turn him in so he could go to jail instead of be shot. Slim tells George that it would be possible if they could find a way to keep Curley from leaving the ranch. Slim says Curley is still upset about his hand and will want to see Lennie die. He then tells George that even if Lennie were taken to jail, he would be strapped down and locked up in a cage. George takes Slim's advice and spares Lennie the experience of being killed by the mob or being locked in prison by shooting Lennie himself.

Why does Slim suggest George to kill Lennie instead of capturing him?

Slim cautions George against allowing Lennie to be captured because he might be locked up and strapped--even caged or worse. "That ain't no good, George." George acknowledges the truth of this observation: "I know."

When Curley and the other men find Curley's wife, they talk excitedly and run for their guns. Curley especially is bloodthirsty, saying, "I'm going for my shotgun. I'll kill the big son-of-a-bitch myself." Hearing this, Slim knows that Lennie will be slaughtered in a brutal way.

Later, after George has found Lennie waiting for him in the clearing, he determines that Lennie cannot be brought in, as Slim has also determined. He shoots Lennie in the back of the head, causing instantaneous death. When Slim arrives, he sits down with George, saying, "Never you mind....A guy got to sometimes." Then Slim leads George to the path entering the clearing and out to the highway, observing, "You hadda, George, I swear."

Shortly thereafter, Slim leads George to the opening in the clearing, where the path resumes, and up toward the highway, offering to take him for a drink. "Yeah, a drink," George replies, remembering the child-like man who was his friend.