Arthur Miller uses decisions and consequences to drive the plot of his play. Joe's decision to lie about his role in the shipping of the cracked cylinder heads has resulted in the destruction of his marriage and his reputation. The denial that Joe and Kate apply to Joe's role in this carries over into Kate's denial of Larry's death. If Joe is guilty of shipping the cracked heads, then he might have caused Larry's death. Ironically enough, we do learn that Joe does cause Larry's death - not because Larry's plane had a cracked cylinder head, but because larry couldn't bear the thought of his dad having been involved in that act.
Likewise, Kate's refusal in accepting Larry's death has resulted in pushing her son Chris away. Even though Chris wants to marry Annie, Kate refuses to accept Larry's death, meaning that his girl is not available for anyone else. This denial of Kate's drives Chris away, to the point where he is ready to move away from his family for good.
Joe's decision to kill himself might have solved all these problems. After reading Larry's letter, Kate can no longer be in denial of his death. Once she accepts his death, Chris and Annie might be free to marry. All of these are inferences one can make after hearing that final gunshot before the curtain closes.