There is really no justification whatsoever for Joe Keller's actions. Over a period of time, he knowingly shipped cracked cylinder heads to the Air Force, which in turn caused the deaths of twenty-one pilots. Had it not been for Joe's unconscionable greed, it's likely that those young men would have lived to fight another day.
And yet, Joe still has to live with himself. He can't hold his hands up and admit to being a greedy criminal whose desire for money led to the deaths of American pilots. So he tries to justify his actions to himself, with varying degrees of plausibility.
His main justification is that he wanted to protect his family and their way of life. The Keller family have done very well out of Joe's greed; they've achieved an enviable standard of living, and Joe is not going to see it snatched away from them.
Less convincing is Joe's assertion that his crimes pale in insignificance with those of others. This may very well be true, but it's frankly irrelevant. Even if other people have committed far worse crimes than Joe, that does not give him the right to commit crimes of his own.
Finally, Joe is haunted by memories of the Great Depression, which was a very hard time for him, as it was for many Americans. Having worked hard to give himself and his family a shot at the American Dream, the last thing Joe wants is to be plunged back into the poverty he experienced during the worst economic crisis in American history.
Though one can sympathize with Joe in this regard, his obsession with avoiding poverty doesn't in any way excuse his wicked actions. For one thing, Joe could still have made a pretty good living for himself and his family without resorting to underhand practices. But he's never fully satisfied with how much money he has; he wants more and more, even if he has to resort to crime in order to get it.