In the final confrontation between Willy and Biff, the two men argue about the conflicts that exist between them. Biff attempts to lay down his side of the dispute, not in reconciliation as much as truce. Willy, however, sees this attempt to end the argument between them as an act of spite.
A natural interpretation of Willy's feelings suggests that Willy is simply hurt by Biff's announcement that he is going to leave and never come back to his parents' house. He is hurt and feels that Biff is hurting him intentionally, so he claims that Biff is acting in spite.
This is not true from Biff's perspective. Linda has told Biff to leave. Biff has also realized that he has been carrying around a fantasy about himself and a grudge about his father for too long. He wants to let go of both and is willing to say that Willy is not to blame for either.
Willy will not accept Biff's truce and refuses to really hear what Biff is attempting to say.
Spite is not Biff's motivation, however, and that is clear. His motivation is quite the opposite. He is motivated by sorrow, penitence, and love (for his mother at least, and maybe his father too).