Arthur Miller gives a good bit of space to the description of Joe Keller in the opening stage directions:
A heavy man of stolid mind and build, a business man these many years, but with the imprint of the machine-shop worker and boss still upon him. When he reads, when he speaks, when he listens, it is with the terrible concentration of the uneducated man for whom there is still wonder in many commonly known things, a man whose judgments must be dredged out of experience and a peasant-like common sense. A man among men. (1.1)
The description of Joe as a simpleton comes up again and again in this play. Chris teasingly calls him an "elephant" (1.535) and Kate calls him a "bull" (3.36). After George's ominous phone call, Kate warns her husband to "be smart now, Joe. The boy is coming. Be smart" (1.620). Miller emphasizes Joe's lack of education as one justification for his criminal actions. We don't think Joe approved those cracked cylinder heads because he's stupid. He approved them because, as an uneducated man, he needs all the more desperately to protect his way of making a living.
Joe has always been concerned with money. With the Great Depression fresh in his memory – and personal poverty even older than that – economic security is his greatest concern. Joe is outraged when Kate and Chris attack him for saving his business. "I spoiled both of you," he says. "I should've put him out when I was ten like I was put out, and make him earn his keep. Then he'd know how a buck is made in this world" (3.63). Joe's narration of his triumph over the criminal justice system concludes with the boast that "fourteen months later I had one of the best shops in the state again, a respected man again; bigger than ever" (1.446). Until he finally understands the cause of Larry's death, his primary value is the success of his business and his ability to make money.
Miller doesn't totally demonize Joe, however. It's not just for his own comfort that Joe makes money; it's for his family. "Nothin' is bigger" than family to Joe (1.67). Though they don't like to admit it, Kate and Chris reap the benefits of Joe's single-mindedness. Kate has a nice house and garden. She can look forward to steak and champagne by the sea. Chris stands to inherit a lucrative business that will similarly support a cozy family life with Ann. We believe Joe when he tells his son, " I did it for you, it was a chance and I took it for you […] for you, a business for you" (2.546, 556). Joe isn't evil, he just has a tragic lack of vision.