I think that Steinbeck depicts the good and the bad in the main characters in Of Mice and Men by showing them to be human. There are no simplistic or reductive elements in his characterizations. Steinbeck has constructed characterizations of human beings. This means that they possess desirable traits alongside not-so-desirable ones. They are human. George and Lennie feature good traits within them such as their loyalty to one another, their willingness to earnestly work, and their ability to treat people as ends in and of themselves as opposed to treating them as means to specific ends. However, George is shown to be irritable with Lennie, sometimes abusing him and disrespecting him. Lennie is a child and, like all children, he gets into trouble. Sometimes, the trouble is fairly destructive. The reality is that both of them are shown to be human. They are filled with good and bad qualities, but Steinbeck creates an empathy within the reader to look at them in the most open light. We simply feel bad for them, understanding who they are - blemishes and all.
Steinbeck carries this dynamic to the other characters as well. Crooks is shown as a human being in his fullest form. He is one who yearns for company, but also has difficulty trusting people. The combination of both yearning and rejection is a part of Crooks' human condition, a way for Steinbeck to show the good and bad in him. Curley's wife is much the same. There is a sadness to her, believing in the hopes of her dreams only to be let down by them. At the same time, this pathetic portrait is balanced out by her cruelty to Crooks, Candy, and Lennie when she confronts them. At that moment, there is little that is likable in her character. The combination of both realities is what makes her so human. Even Slim, a character that earns Steinbeck's respect, is shown to be human. He is the consummate leader on the ranch, but when the group intending to find and harm Lenny is formed, Slim does not speak out and appeal for calm. He does not join the group as an active member like Carlson or Curley. However, Slim is human. He demonstrates both good and not- so- good within him. Steinbeck seeks to make a statement that in showing people as they are - capable of both destruction and restoration - human beings are complex and intricate.