For most of us, our families are the most important things in our lives. We love them, cherish them, and will do everything within our power to protect them from harm. Sometimes, in extreme circumstances, this will entail doing things that, under normal conditions, we wouldn't do. And such things are often bad.
In essence, this is Joe Keller's justification for shipping faulty plane parts, cracked cylinder heads, to the Air Force. He thinks that by cutting corners in order to make as much money as possible, he's actually doing the right thing for his family. He wants them to be financially secure, to enjoy a consistently high standard of living.
But Joe's self-serving justifications for his unconscionable actions are completely bogus. For one thing, these are not extreme circumstances we're dealing with here. Joe has not been forced into doing what he's doing; he didn't have to ship those faulty plane parts to the military.
Joe had a choice, and he made the wrong one. Besides, his family were not at any serious risk of ending up on the streets if he'd done the right thing. They would still have enjoyed a good living.
In the end, Joe's actions didn't protect his family after all. Because of his greed, his son Larry committed suicide out of shame when he realized what his dad had been up to. In turn, Larry's death has caused incalculable suffering to the Keller family. Far from being excusable in the name of family, the wrongs that Joe has committed have proved to be anything but.