Sportswriter Al Stump is hired in 1960 as ghostwriter of an authorized autobiography of baseball player Tyrus Raymond "Ty" Cobb. Now 73 and in failing health, Cobb wants an official biography to "set the record straight" before he dies.
Stump arrives at Cobb's Lake Tahoe estate to write the official life story of the first baseball player inducted into the National Baseball Hall of Fame. He finds a continually-drunken, misanthropic, bitter racist who abuses his biographer as well as everyone else he comes in contact with. Although Cobb's home is luxurious, it is without heat, power and running water due to long-running violent disputes between Cobb and utility companies. Cobb also rapidly runs through domestic workers, hiring and firing them in quick succession.
Although Cobb is seriously ill and prone to frequent physical breakdown, he retains considerable strength and also keeps several loaded firearms within easy reach at almost all times, making the outbreak of violent confrontation always an immediate possibility in his presence.
Cobb almost gets killed in an automobile accident off the Donner Pass, driving recklessly in a blizzard. Stump rescues him, but Cobb then seizes control of Stump's car until he gets into another accident. The car has to be towed to Reno.
Stump and Cobb go see a show at a Reno resort hotel featuring Keely Smith and Louis Prima, whose act Cobb rudely interrupts. A cigarette girl, Ramona, becomes interested in Stump, but when Cobb barges into the hotel room, he's in a jealous rage. He takes Ramona to another room, where he physically abuses her.
Cobb and Stump travel together cross-country by automobile to the Baseball Hall of Fame's induction weekend in Cooperstown, New York, where many star players from Cobb's era are in attendance, including Rogers Hornsby and Mickey Cochrane. Cobb is haunted by images from his violent past as he views film footage of his career.
From there, Cobb and Stump drive south to Cobb's native Georgia, where his estranged daughter continues to live. She refuses to see him. Stump, having spent months with Cobb witnessing his behavior and absorbing considerable abuse, is torn between creating the autobiography that Cobb hired him to write and writing his own book on Cobb's true self. Cobb begins to regard Stump as a friend of sorts; it is clear his conduct has driven away virtually all his legitimate friends and family.
Stump writes two books simultaneously: the one Cobb expects, and his own, sensational, merciless account which will reveal the real Cobb, warts and all. Stump plans to complete Cobb's version while the old man is still alive, guaranteeing his payment for the project, letting Cobb die happy, then issue the hard-hitting followup after Cobb is gone. After a long night of drinking when his own personal life begins to unravel, Stump passes out. Cobb discovers his notes for the no-punches-pulled version, bringing on an epic explosion.
Cobb begins to cough up blood and is taken to a hospital, where he wields a gun and treats doctors and nurses as harshly as he has everyone else. Stump gains a grudging respect for the player's legendary intensity and fearsome competitive fire, as well as an understanding that the murder of Cobb's father may have been partly responsible for his antagonistic personality. Stump is conflicted in his opinion of Cobb, and, in the end, completes the glowing autobiography Cobb hired him to write.

Question: What does Mr. Stump discover that he thought may have been partly at the source of Mr. Cobb's antagonistic personality? 
Answer: The tragic murder of his father.