The most redeeming quality in Willy is his concern for his family. Though he is deluded, stubborn, difficult, judgemental, unfaithful, and unfair, he does strive to succeed for the sake of his family as much as he seeks success for himself.
Willy wants to see Biff succeed and he, for a time, truly believes in Biff. There was great love between father and son while Biff was still a boy. In his suicide, Willy hopes to leave his family with some money from his life insurance. His idea is that though he was a failure in life, he may be a success in death.
In his suicide, Willy robs his family of his continued presence and emotional support, but he hopes to have given them something he never could in life - the advantage of money.
Though his ideas may be deeply misguided, they are motivated by an impulse to provide for his family.