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BIFF: Exactly what is it you want from me?
WILLY: I want you to know on the train, in the mountains, in the valleys, wherever you go that you cut down your life in spite.
BIFF: No, no!
WILLY: Spite, spite is the word of your undoing! And when you’re down and out remember what did it. When you’re rotting somewhere beside the railroad tracks, remember, and don’t you dare blame it on me! (Act 2)
Willy assumes that Biff's betrayal of his expectations is intended as a punishment for his betrayal of Biff's trust when Willy had an affair. Here, Willy tries desperately to separate himself from his own guilt about both Biff's failure in life and the reality of his infidelity.
BIFF: There’ll be no pity for you, you hear it? No pity!
WILLY [To Linda]: You hear the spite!
BIFF: No, you’re going to hear the truth—what you are and what I am!
LINDA: Stop it!
WILLY: Spite! (Act 2)
Willy's accusation that Biff is spiting him stems from his knowledge that his son felt betrayed by his affair.