Well i think if you asked Steinbeck, he would probably say that it was the only solution. And i think i would have to agree with him. If George had not killed Lennie, and rather, had ran away with him, the same thing would have happened over again. As witnessed by the puppy and bunny that Lenny crushed, Lenny simply didn't know his own strength. Since neither George nor Lennie had any money, they would have had to stop and work, and be around people, and Lennie would have gotten in trouble again. Obviously given Lennie's temperment, George couldn't leave him. The only other solution, then, was to have the other men find Lennie and shoot him themselves. However, in that scenario Lennie would have been frightened and confused. This way he died thinking about the thing he wanted most in the world: his own farm with george.