I have very few fresh insights, and I blame this on the book itself. It is not a book that wants to be reconsidered. Consider the too-neat symbolism: In the book's middle section, an old dog is put down to spare it suffering. At the end, the novel's protagonist, George--spoiler alert, if you need it--does the same to his friend Lennie, to save him from a crueler lynching at the hands of the other migrant workers. (Lennie, exceedingly strong but with the mind of a child, has accidentally killed a woman because he wanted to pet her soft hair.)
The only real question of significance Steinbeck offers is this: Does George make the right decision? I believe that Steinbeck wants us to think so ("Slim said, 'You hadda, George. I swear you hadda.'") But I do not think it is in keeping with George's wily, calculating character to give up so easily. As such, the premise of the question strikes me as faulty, and invalidates the question.
I noted in my old review that this final scene has "great resonance," and that remains true (though I might excise great). Like George, we are meant to understand that Lennie has been somehow spared, and so George himself receives the brunt of our pity. Lennie was the only constant in George's life:
George went on, "With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us. We don't have to sit in no bar room blowing in our jack jus' because we got no place else to go. If them other guys gets in jail they can rot for all anybody gives a damn. But not us."
Lennie broke in. "But not us! An' why? Because... because I get you to look after me, and you got me to look after you, and that's why." He laughed delightedly.
George's act of questionable kindness is perhaps cruelest to himself, as it leaves him alone--and after all, the migrant workers of the novel work near the California town of Soledad, Spanish for "loneliness." But it is also possible that soledad is a state that George and Lennie have always occupied, given their natural isolation from each other and inability to connect. Can George and Lennie possibly understand each other; can they know each other, or sympathize with each other in any meaningful way? In this way their relationship mirrors the chaotic, deracinated lifestyle of the Depression-era migrant worker, in which all relationships are by nature transitory and any connection impossible, whether it be to man or to place. We find an echo of this in the ranch that Lennie and George dream about, which will go unrealized: another abortive attempt to form a bond.
This would be a cruel reading, and I have serious doubts that it's the one Steinbeck intends. But I think it would be a hell of a lot more interesting.