Willy has conflicting images of Biff, but why does he defend and criticize Biff?
I have to answer this question in essay form and I'm having a hard time fulfilling the length of my answer. Any imput is greatly appreciated!
Willy Loman viewed himself in Biff. One must remember that Willy did not have a good childhood nor a good relationship with his own father, nor with his brother. Hence, Biff represents Willy's chance to perhaps build a relationship with his own son, and to be a better father.
So far, Biff had done everything Willy wanted: Football player, mischievous boyish behavior, jock, bully, and Dad's number one son. However, after Biff found out that his father had been cheating on his mother, all of Biff's hopes went away considering that his Dad had fallen out of a pedestal.
Hence, in came the conflict between a father and son that once were cohorts and now are enemies. Both father and son were enmeshed in a very dangerous co-dependent relationship in which one fed off from each other. This is the reason why Willy both insults Biff for not reaching up to his former hopes ans goals (which was Willy's own fault) and he also praises him but in his former self: As the Biff that would have been the successful man and well-liked football player who once looked up to Willy.