In Of Mice and Men the characters consists on rogue, lonely, wandering men from the Great Depression who, due to the situation that the world is in, have to make due with whatever way they can find to make money.
In the farm, they get a place to live, payment, food, and even a shot at camaraderie. However, each man is plagued by his own past, or by his hopes for the future. Whatever side is taken, whether that of good or evil, the men pretty much all live in the daily purgatory which is Soledad.
Each character would be representative of some weakness or flaw in the everyday man. Let's look at them from both sides of the spectrum.
George- George is the embodiment of kindness, regardless of how he treats Lennie. He has known Lennie from a very young age, and has chosen as his path of life to remain with Lennie as a protector. He claims that Lennie is his cousin for the sake of defending him from Curley. He even defends his friendship and protection of Lennie when he speaks with Slim. When George is "too rough" with Lennie, he is sure to acknowledge it
I been mean, ain't I (Ch. 1)
He also continues to bring up the dream that he and Lennie have built for themselves by saying "they are not like us", meaning, George and Lennie have each other. Even his choice to kill Lennie at the end was a sacrifice born out of morbid kindness: He would have rather killed his friend mercifully than allow him to be lynched by an angry mob.
Lennie - Lennie represents the basic Id proposed by Sigmund Freud. That primitive and impulsive nature in all humans that renders us wild and uncontrollable. As much as he tries his (self-monitoring) Ego to kick in, his cognitive dissonance makes him unable to regulate his emotions and actions. That being said, Lennie is unable to be good or bad, because those are choices. He can only be Lennie, and respond to what his nature dictates his body to do.
Slim - At the center of it all, stands Slim. Balanced, focused, even handsome. Slim is the epitome of the man that is "put together" and is aware of his surroundings, the people about him, and himself. He is so balanced that George feels comfortable opening up to him. Curley does not even dare to mess with Slim either. He is the balancing force among the men.
Candy - Represents subjugation. He cannot make choices either, as he is a slave of his own life. He is someone who needs support at all times. He needs the company of his sick, old dog, or Lennie and George, in fact, anything would do. Candy is too dependent on everything to be considered a bad person; that would require initiative. He has waited long years, knowing that he has some money in the bank, living on the shadow of the ranch. Makes one wonder, why did it take for Lennie and George to go there for him to awaken to a better potential life?
Crooks- Vindictive, cynical, cruel and antisocial, Crooks has at least one excuse to be each and every one of those things, twice over. He is a victim of circumstances that range from physical, to financial, to social, to racial. Anyone in his situation would become a "Crooks". We know that his behavior does not stem from malice, but from deep pain. We know as much when he buys into Lennie and George's dream and asks to be allowed to work with them, even if it is for free.
Carlson- Carlson represents "blind America", as in oblivious, clueless, and careless. He does have a streak of sociopathic behavior that manifests in his absolute disconnect with the emotions of others. Concerned only for his own opinion of things, he may even be narcissistic. The non-challant way that he demands that Candy's dog is killed, and the carelessness with which he regards the life of the dog may even upgrade him from sociopath to psychopath. That would still be far-fetched based on the few things we see him do, but Carlson is not a good man.
Curley- Curley is even worse than Carlson because he KNOWS when he is hurting people , but does it anyway. He takes pleasure in abusing others. He is a bully, insecure, and an overall small, coward man with a complex. As the son of the owner of the ranch, he feels that he is above everyone. Therefore, Curley and his father represent the very real culture of nepotism that permeates so much of our society.
Therefore, there are not so many black/white characters. There is a degree of complexity with each of them that makes them quite human, thus impossible to judge under the parameters of "good and bad". There is, after all, good and bad in every one of us.