This is a difficult question to answer, because many will say that George had no choice. Just from the text, we can say that George thought this was the best option. Slim, who is seen nearly as divine in the book, agreed. He said:
Slim twitched George’s elbow. “Come on, George. Me an’ you’ll go in an' get a drink." George let himself be helped to his feet. “Yeah, a drink."
Slim said, “You hadda, George. I swear you hadda. Come on with me.” He led George into the entrance of the trail and up toward the highway.
If I were to argue that George made a mistake, I would make two points. First, George actually murdered Lennie. Murder is never the right answer, because George actually did not know what would have happened. Perhaps, Slim could have persuaded the men that it was a mistake. The odds might have been slight, but there is always a chance. But when it comes to death, it is always final.
Second, George and Lennie had a plan. So, all along George had an idea that things might not work out. In other words, he knew that they might have to flee. So, George had an option. More importantly, Lennie was waiting for him, and they could have ran away. If all else failed, they could have suffered together. In this way, George would have lived his life with no regrets and without a guilty conscience.