Biff's choice at the end of the play (to face the reality of "who he really is" and to drop his false pretenses), to me, shows that free will was the determining factor for all the characters.
Willy's problem was that he could not or would not see himself clearly and honestly. As a result, he dreamed a dream that was impossible - for him. This dream was not of his own creation, but it was within the scope of choice for him to change his relationship to the dream. Biff makes that clear.
I think this is one of the essential questions the play begs readers to ask of themselves. Are these characters innocent bystanders playing out a part that fate has already written for them? Or, are the choices Willy makes throughout his life--his limitless idealism, the refusal to acknowledge failure and set-backs, his re-writing of past history--bad decisions that reap repercussions affecting not only his life but also the lives of those around him? Personally, I'm more inclined to go with the latter belief.
A difficult question that arises with regards to many texts (i.e. often with Shakespearean plays)--is fate or free will responsible for the actions and lives of the characters? In stories, fate often presents characters with a series of options or forking paths in their journeys. This is true in Death of a Salesman: Willy has the option of taking a job from Charley, traveling farther away, or settling into a routine. Willy chooses none of these options, however, and continues to struggle with his unachieved dreams. In this respect, Willy's life is guided by his own free will. So fate and free will work in conjunction and affect the lives of characters.
I agree that Willy's life (and death) and the futures of the other characters are ultimately affected by free will. I choose to believe that and see that is my general outlook on life. I don't necessarily think that is what Arthur Miller intended -- I think he was more focused on the many factors which contributed to Willy's demise (in a sense Willy is set-up from Miller's perspective) and that his death was an act of courage because it was the last thing he had to give. I think it is an interesting perspective and could very well be what Willy (if he were real) intended his death to be -- a gift and an act of penance. However, just because Miller created the character, I think my life perspective is still valid. I also recognize too that it would have been very, very difficult (and unlikely) for Willy to change his path. Without some incredible moment of clarity, followed by a lot of hard work, Willy is on an almost inevitable path to destruction. I like to think that Biff "gets it" and performs the 180 that Willy was unable to achieve. I think it is likely that Happy and Linda live on in ignorance (or self-deception, one might even argue), Happy's continuing to pursue the elusive and false dream of his father, and Linda's vindicating him.
i think that there is no different between fate and free will,we have the freedom to act in life but all our actions will be within the order of fate maybe it is avery complicated thought but it is the fact.