Is Biff and Happy being needlessly curel to their father?
No. In fact, the boys' behavior toward their father is not born out of malice nor cruelty. It is born out of self-realization and the need for change. The treatment that Biff and Happy give their father is almost expected and, to a point, necessary.
In Arthur Miller's play Death of a Salesman, the characters of Willy and his sons, Biff and Happy, represent what is known in psycho-dynamics as an "enmeshed lot". This is a cluster of people who live together and depend on each other in ways that are beyond what is considered cognitively healthy.
Basically speaking, Willy visualizes himself in his sons. For this reason, he meddles in their business as they grow up, and intensely prescribes them with ideas and philosophies that they should follow. He emasculates his kids by being overly possessive of them in the aim of making them popular, well-liked, and successful.
As a result of this, the boys lose their character and personality and fuse them within that of their father. They do as he says, think the way he thinks, and act the way he acts. This is why, as adults, neither Biff nor Happy have found a true purpose in life. They cannot. Their father is used to doing that for them. They are two lose boys already slammed into adulthood.
Therefore, to answer your question, being an enmeshed lot involves the combination of three persons into one same personality. Willy, Biff, and Happy are co-dependants in their need for a purpose, and they are each other's enablers when they allow each other to continue fantasizing about who they really are.
When they fight, they are not being overly cruel. In their case, those behaviors are a cry for help. Biff already notices that his father basically fills their heads up with fantasies that both brothers always believe. Now that he notices, he wants to break free from that sick pattern and tries his best to do the same with his Dad. When Willy clearly refuses to let go of his dream of making it big, Biff retrieves not without first using all he has in his power to snap his father out of it. Nothing works, in the end. Willy commits suicide and still believes that his funeral would be as grand as that of Dave Singleton.
For these reasons, the dynamics of the Lomans do not make their reactions cruel but neccessary. Someone has to say "stop" to the insanity that this family continuously feeds itself. Someone has to speak up whether it is a harsh truth or not. It is one of those things that, simply, needs to be done.
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