What is the overall message of "Master Harold" . . . And the Boys, and how does that relate to society?

I think there are a number of potential overall messages present in this play, and individual audience members will gravitate in slightly different directions regarding what the specific, overall message is. I think there is a message present in the play that teaches audiences about the potential dangers of speaking out in anger. Hally turned his anger over his mother and father back at Sam, and Hally said some very hurtful things. Hally eventually strained the relationship to the breaking point, and Sam warned Hally not to go any farther. Hally did, and the relationship was destroyed.
There's a message there about how to treat other people. There's a message there that shows how a single outburst can damage a lifelong relationship. The play also has a message to audiences about what a loving father looks like. Unexpectedly, Hally's father is not that ideal image. It is Sam. Sam pushes Hally to be better, he protects Hally, he jokes around with Hally, and so on. Sam fills the father role in Hally's life, and audiences are taught what a healthy father/son/adult/child relationship can look like.