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Slim tells George that he had no choice. If Curley and the mob somehow had not killed Lennie, he would have been locked in a cage. Here is a quote for your second question,
He (Slim) led George into the entrance of the trail and up toward the highway.
Curley and Carlson looked after them. And Carlson said, "Now what the hell ya suppose is eatin' them two guys?"
Carlson can't understand the pain that George must feel.