ack Mac Gruder is the central antagonist of Gramps, a 1995 made-for-TV movie also starring Griffith in the title role.
Jack Mac Gruder is a grandfather whose outwardly sweet side is a cover for a bloodthirsty sociopath who nearly succeeds in destroying his family all to get at and rape his "precious" 7-year-old grandson, Matthew.
About 40 years earlier, Jack was the autocratic head of his household, and constantly comes up with new ways to beat his poor wife when she steps even a millimeter out of line; finally, the wife is able to build a backbone and leave, taking her 7-year-old son, Clark, with him (the wife wins the divorce, cleaning out Jack who got no visitation rights to see his son).
In the present, Clark tracks down his father and — hoping he's changed and gotten his anger issues under control — invites him to stay. Jack charms everyone with his guitar picking, but eventually, this sinster pedophile licks his chops as he unleases his pent-up anger on innocent people: he breaks the housekeeper's legs with a baseball bat, smashes a fire extinguisher over the head of a police officer, runs down a teen-aged girl with his car (and also does the same to his daughter-in-law), and kills his daughter-in-law's father after he gathers evidence to refute Jack's claim that the daughter-in-law was cheating on him.
In the end, Jack dies as he lived: Kidnapping Matthew and one of his female classmates, there is one final confrontation along a river, and he threatens to throw the children over the edge of a waterfall if Clark does not pay a $1 million ransom. Clark agrees, but throws the money in the river to the girl. When Jack tries to go after the girl, she lets him have the money and begins throwing out bills into the river; the greedy Jack begins collecting the bills ... until he is caught in the current and unable to avoid going over the edge of the waterfall. Clark rescues Matthew and the little girl as Jack is crushed to death on the rocks below.