If he didn't shoot him, Lennie would end up in the worst of situations. He would probably have been lynched, or is George ran away with Lennie, Lennie would have ended up hurting or killing someone else by accident. Another metaphor that pertains to Lennie's situation is that bad things always come in three. The first bad thing that happened was Lennie killed the mouse, the second, he killed the pup, and the third, he killed Curley's wife. It also builds in the seriousness of the deaths, form the mouse, to Curley's wife.
Another theme that Steinbeck presents to us is the dreams of men going awry. An example of this includes the dreams of Lennie, George, and latter Candy of owning their own land. It all seems to be going great for them, and it seems as if their dream were actually going to become reality. But just when they get their hopes up, something goes terribly wrong, aiding in the prevention of them obtaining their dreams. Lennie kills Curley's wife, and George is forced to shoot him. Another example of this theme, is the disabled men in the story: Lennie being mentally slow, Candy missing a hand, Crooks with his crooked spin, and latter Curley with his crushed hand. These characters show how no matter how much you want to live your life one way, terrible things can go wrong, preventing you form fulfilling your dreams.
Another theme that Steinbeck presents to us is being alone. All of the characters, besides Lennie and George travel alone. "Hardly none of the guys ever travel together. I hardly never see two guys travel together. You know how the hands are, they just come in and get their bunk and work a month, and they quite and go alone. Never seem them give a damn about nobody."" (Steinbeck: 39) All of the other characters see the pair as unusual, and at the same time they think the idea of traveling with a companion nice. "S'pose I went with you guys. Tha's three hundred an' fifty bucks I'd put in.