In examining the character of Joe Keller, it is best to see him as a fundamentally decent man brought low by a tragic flaw. In his case, that particular flaw happens to be greed. Joe has realized that he can make a lot more money by cutting corners, selling cracked plane cylinders to the Air Force.
If Joe had done the right thing, he would still have made a decent living for himself and his family. But because he was so blinded by greed, he was prepared to sell his soul to the devil and ship what he knew full well to be faulty plane parts to the military.
Is Joe really no worse than other men, as Chris would have us believe? Perhaps not. As we've already argued, he's basically a decent man brought low by greed. But that doesn't in any way let Joe off the hook. Many would argue that his actions were evil. Joe knows that those plane heads are faulty; he knows that they can lead to death and serious injury. And yet he still sells them to the Air Force, all because he wants to make more money.
Reaching any degree of moral consensus concerning what's right and wrong is notoriously difficult. But it's fair to say that most people, when presented with the facts of the tragic case of Joe Keller, may regard his actions as evil.