I would have also encouraged Lennie to fight back against Curley. Curley was asking for it! When Lennie crushed his hand, it wasn't Lennie's fault - it was Curley's own fault. George must have known that Lennie could have done worse to Curley and he encouraged Lennie to fight back anyway.
If Lennie had killed Curley, I think we would still have a hard time blaming Lennie for the act, just as it's hard to blame him for crushing Curley's hand.
I do not feel that I would have the fortitude and strength of character to shoot Lennie, but I would have told him to fight back when he was being attacked by Curley. I think I would have done so from instinct, as the bond between Lennie and George was so strong it would be agonising to see him being so viciously beaten.
In George's place, I would not have killed Lennie. I know that is not what you asked, but the logical next step would be to choose an action that I would not replicate. I agree that we will never be in the exact situation, but if I was I would try to find another way to save Lennie or left him to his fate. George needs to go on with his life.
There was some personal identification with George in his inspection of his bunk and the area where he was to have put his things, along with his skepticism about the former tenant. And, when Candy speaks of the stable buck,George's asking about him rings of a similar quality as well as his inquiries about the boss. Knowing as much as possible about one's new job and situation is always a good idea.
This is a difficult task. In Of Mice and Men, George is a praiseworthy character in many ways. Choosing one action done by a literary character and saying I (or any of us) would do the exact same thing is probably impossible. We're all different and are driven by different motivations even when placed in exact or similar circumstances. One thing I probably would have done in roughly the same way as George is try to take on the responsibility for Lennie. As a woman, I'm not so sure I would have been very successful in that place and time of taking care of him in the way George did. There would have been fewer options open to me to make a living on my own, let alone with Lennie by my side; and the simple logistics of taking care of us both would have have been much more complicated. In short, then, I would have done my best to care for Lennie just as George did, even though my doing so would probably have looked quite different than how George did it.