Why did the Old Man kill his own father?
The Old Man's father was a dissolute wastrel, a chronic alcoholic who shamelessly frittered away the family fortune. As a boy, the Old Man was prevented by his father from attending school, and so to this day, he still harbors resentment towards him for his ignorance. The Old Man's son seems unconcerned by his late grandfather's character. Rather, he appears more interested in the glittering world of wealth and privilege his family once inhabited.
At the same time, the young man is aware of certain unpleasant rumors concerning his father, the Old Man. One night, the Old Man's father got drunk as usual, only this time he burned down the family home. The boy has heard that the Old Man killed his father that night. When asked about it, the Old Man admits that the rumors are indeed true.
It would appear that the Old Man was motivated primarily by revenge. His mother married beneath herself and the result was the destruction of a great family and its ancestral home. The Old Man is also eaten up with resentment over his reduced status in life, directly caused by his father's profligacy. Although he couldn't have foreseen how his life would turn out when he stabbed his father to death amid the burning wreckage of the family home, the years since have confirmed to the Old Man's satisfaction that he made the right decision.