Why does Slim tell George that it's a bad idea to capture Lennie instead of killing 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.