Explain why, in Chapter 6, Slim tells George, "You hadda, George. I swear you hadda."

I think Slim knew what George actually did, and that it was George who stole the Luger and killed Lennie. He probably can also understand the great amount of emotional stress George was going through after he killed his friend. Therefore he was comforting George, telling him that he did the right thing, and that even if he did not kill Lennie, Curly will.

:) By the way, Aslan, are you a teacher?