In a society in which ranch-hands and migrant workers generally travelled alone, George and Lennie, the protagonists in Of Mice and Men, stay with each other as they move from place to place looking for work. At the beginning of the novel, readers quickly understand that Lennie is mentally handicapped, and that he relies on an often resentful George for many of his basic needs. While Lennie loves George and eagerly looks forward to achieving their goal of owning a farm and living "off the fatta the land," George often sees Lennie as a burden who keeps George from having the kind of life he feels he deserves.
In a telling conversation with Slim in Chapter 3, George explains that he used to play tricks on Lennie in the past but has since stopped doing so. He maintains that Lennie isn't stupid--he's just simple--and explains that Lennie would do anything George asked him to do. Lennie's unwavering loyalty to George eventually caused George to realize the error in his ways, and George's ultimate commitment to Lennie is evident throughout the rest of the novel.
Although George is often frustrated with Lennie (he yells at him, becomes angry when Lennie asks him to repeat things over and over, and becomes upset when Lennie doesn't follow directions), readers recognize that the two characters have a mutual love for each other. They share a common dream of owning land, and they've become so accustomed to each other's company that it is difficult for readers to imagine one without the other.
The ultimate example of George's love for Lennie comes at the end of the novel, when George kills Lennie to save him from Curley. Though it may seem impossible to imagine a situation in which killing another human being is an act of love, given Lennie's history and potential future (a painful death at the hands of Curley and the others), George knows that killing Lennie is actually an act of mercy.
In his conversation with Slim, George talks with the man who has "God-like eyes fastened on him." Feeling comfortable with Slim, George explains that he and Lennie were from the same town, so he knew Lennie's aunt with whom Lennie lived. As George reflects on how he used to make fun of Lennie, he remarks,
"Used to play jokes on 'im 'cause he was too dumb to take care of 'imself. But he was too dumb even to know he had a joke played on him. I had fun. Made me...seem smalrt alongside of him...."
But, George continues, "in a tone of confession," his ridiculing of Lennie ceased being fun because Lennie never became angry; in fact, Lennie even let George hit him without doing anything. When George wanted to show off one day before others, he told Lennie to jump into a river. Obediently, George did so, and nearly drowned because he cannot swim. Nevertheless, when George and the others fished him out, Lennie was "so damn nice," having already forgotten that George told him to jump: "Well, I ain't done nothing like that no more." For, George realizes that for all his size and strength, Lennie is a child and innocence of any complex motives that others may have. George realizes that Lennie needs someone to protect him from the cruelty of others.
This conversation of George with Slim illustrates the theme of the importance of the fraternity of men as a force against the terrible alienation of men during the Great Depression.
To me, what this tells us about George is that he does have a mean and cruel streak in him. However, it also tells us that he is at least trying to rein in that tendency.
You can see where a person with a bit of cruelty would have a fun time messing with Lennie. He is so stupid that it is easy to play jokes on him. It is the kind of thing that could really appeal to a young man, especially one who has a bit of a cruel nature.
But the fact that George says he has quit shows us that he is trying to reform. Just from what it says, you would have to say that he has reformed because he says he no longer plays jokes on Lennie. But we know from the rest of the book that he still will pick on Lennie some and make him feel bad. This shows me that George has not completely shut down that side of his character.