My thesis is "In Of Mice and Men, George was morally justified for shooting Lennie for the reason that his motives were acceptable. George may have taken Lennie's life but he did it for the greater good of his friend. He knew that Lennie would never truly be happy again if he was locked in jail and therefore decided that Lennie would be better off this way."
As post 6 points out, the second half of the opening sentence is a bit redundant.
You suggest that Lennie may have been put in jail if he were captured, but the characters in the novel clearly state that Lennie would be put in a "boobie hatch", a home for the mentally ill or mentally incompetent.
With this aside, you have chosen a clear and interesting thesis.
One question that might lend another side to your essay is one that asks if George was acting for the benefit of the community (and not just Lennie's best interests) when he kills Lennie. After all, Lennie is dangerous.
Discuss how George was morally justified in shooting Lennie.
Lennie could be justified in a number of ways to shooting Lennie. First, George knew that the other ranchers (namely, Curley) were going to kill Lennie themselves. George was not comfortable in the fact that another man was going to take Lennie's life. He had taken care of Lennie to this point and he was going to continue too, even if it meant killing Lennie himself. Secondly, Lennie had taken the life of another person, Curley's wife. George had justification in taking Lennie's life as retribution for the murder Lennie had committed. Lastly, Lennie would not be able to survive on his own. Curley had told George to "stick with us so we don't think you had nothin' to do with this". George had no choice. He had to take Lennie's life so that blame did not fall on him as well.
Long ago there was a movie whose setting was, like Mice and Men, the Great Depression. The title of this movie was They Shoot Horses Don't They? and the plot centered around the desperate lives of people duringthe depression of the 1930s. The central action of this film revolved around the marathon dances held at the time in which people could win money if they were the last couple standing. Poor people endeavored, therefore, to dance for hours and hours in the hope of attaining money with which they could eat. However, many of them collapsed and suffered greatly in their efforts. Their pitiful actions and miserable lives were portrayed in detail; their situations were so painfully tragic that, finally, one dancer, weakened and sick from hours on her feet asked the man who held the money, "They shoot horses, don't they?" But, people must bear their terrible misery.
Is he morally justified? Would a man not shoot any other animal that suffers? It is no coincidence that Steinbeck uses animal terms for Lennie, and that he precedes Lennie's death with the death of Candy's old dog.
George chose to keep Lennie from bearing a terrible misery. Lennie, who has been described in animal terms, is shot as a horse is shot: to put him out of his misery.
I don't think George killed Lennie to achieve "the greater good for the greater number of people." I agree with the others who recognize that George understood he could not save Lenny from this last tragedy. There was no escaping the consequences. George was not so much concerned with what the law would do and how that would affect Lenny as much as he was what the men would do to Lennie.
Just think about what would have happened to Lennie if George didn't kill him. Would things have been better for Lennie? I doubt it.
So what George is doing is saving his friend. He knew that Lennie was going to be killed and probably in a way that would be really painful both physically and emotionally for Lennie. (I'm assuming that Curley would have beaten/tortured Lennie.) So instead, George kills him before he knows it, and he makes sure that Lennie is happy before he shoots him. It's the best possible thing for Lennie in that situation and so it's the morally right thing to do.
I think that there are some elements from the novella that can be used to help justify your thesis. I would point out how George understands Lennie's dependence on him. Part of your thesis is making the argument that George's care for Lennie is what compels him to shoot him. This indicates that George recognizes that no one else will care for Lennie as he will. He speaks of this to Slim in chapter 3, where George tells him that Lennie would do anything he says. As a joke, George told Lennie to jump in the lake, and he did. George conveys to Slim that he understood the importance he has to Lennie and he would not abuse that again. That demonstrates how George recognizes that the bond between them is on him to maintain. At the same time, I would also point out to the ending. Steinbeck goes through an intense level of discussion in terms of the pain George undergoes in raising the gun, telling Lennie about the farm, and pulling the trigger. It does not seem consistent that George would endure this agony on an almost intestinal level if he did not feel that there was no other option for his friend. Finally, I think that you could point to the end of Chapter 5, when George recognizes the truth of Slim's words when he says that Curley is looking for Lennie strictly to kill him. George tries to mount a defense, but he recognizes its frailty. The fact that he takes Carlson's gun proves that he understands that killing Lennie is the only option he has out of mercy, love, and compassion for him.