In what sense are Willy's dream and his unethical behavior causes of his tragedy in Death of a Salesman?
I am not convinced that any of Willy's behaviour was 'unethical'. He tried to succeed in the sales business using the preferment and loyalty system which no longer exists in the world of business. Perhaps his plans to commit suicide with the pipe he places by the boiler are thoughtless. Having suicidal thoughts, and acting on them, may be seen as a sin by some. His desparate clinging to the American Dream and all it promised is certainly the tragedy.
I'm not sure what you are referring to by his "unethical" behavior. The only thing he does that might be labeled unethical is/are his fling(s) in Boston ... getting caught by Biff changes their relationship and probably their whole life. Biff and his achievements, significant or not, and his "pal" relationship with Willy were critical to Willy, and it all ended when he got "caught" in Boston. Some people might call Willy's behavior unethical; some might look at it as just a weakness. Whatever your position, it is getting caught in this compromising situation that creates all the damage.
The American Dream part may be simpler. Willy believes that being "well liked" is the key to success in business, and that success in business would lead to happiness in life. He had a loving wife and two adoring kids (although he didn't pay too much attention to Happy), and could have been happy if he didn't have to be the greatest salesman in the world. The American Dream is about freedom, not economic success; would that Willy had seen this.
We all like to believe that humanity is the pinnacle of civilization, but what Athur Miller's play creates is a parallel with Christs sacrifice. Willy Lomam's sacrifice to his family can be compared to Jesus' sacrifice on so many levels. Firstly Jesus sacificed for humanity whereas Willy sacrificed for his family (today's society regards immediate family paramount above all else) Capitalism is the modern religion and Miller is highlighting the failings of money over spiritualism. Capitalism cannot provide adequate comfort to life as much as religion can.
He is so caught up on his own goal of pursuing the American Dream that Biff cannot stand him anymore and his obsession with trying to be successful coupled with his fear of not being respected is a recipe for disaster. Ben serves as Willy’s symbol of success. Whenever Ben is in known he sucks up to him a lot and when Ben says that he’s going to be late he basically begs him to stay longer. His sons also abandon him at the restaurant after he gets sick in the bathroom. His need for approval eats him alive. His jealousy of Charley is evident when he doesn’t accept his job offer, he doesn’t want the help from others; he just wants to be acknowledged, but when his whole life is based on something he can never attain, he loses all meaning of his life and decides to end it.