Joe for a long time is able to use rationalizations, justifications, and some magical thinking—such as saying he didn't think the faulty cylinder heads would ever end up in the planes—to avoid accepting blame for his part in the scandal. He says he did it for Chris, rationalizes that he is doing nothing worse than any other man, and refuses to link the faulty parts to Larry's death.
By the end of the play, however, his guilt has been exposed to his son, who is not accepting any of his father's dodges. Chris is determined that Joe accept the reality of what happened and own his part in it. Finally, he reads aloud Larry's letter. In this letter, Larry writes that he is taking his own life because he knows his father is complicit in selling the cracked cylinder heads that led to the death of the twenty-one pilots.
Jolted into the awareness that his actions, meant to protect and help his family, led directly to the suicide of his own son, Joe finally feels the weight of the twenty-one pilots. They go from being disembodied statistics to flesh-and-blood human beings just like Larry. They are, as Joe states, "all my sons." At this point of empathy, Joe feels intense guilt. He commits suicide by shooting himself.