When Slim says "A guy got to sometimes," right at the end of the book, he's referring to George shooting Lennie. He's saying not that a man has to shoot another man sometime—that rarely happens—but that a man sometimes has to step up and do the difficult thing. This is foreshadowed earlier in the book when he nudges Carlson along in killing his old dog. It's a recognition of responsibility, and of how hard it is to do the ethical thing sometimes. He's trying to consol George in a difficult time.