Regardless of how insane this beef is to begin with based on what's shown so far...
Taking it out on the descendant of your rival after he is long dead is not normal.
I don't recall any other instance where any character, fictional or real does this.
Competition and conflict and rivalry is has been with us forever we all know that, but when does it ever involve their loved ones or descendant?
The few I can think of:
1. They're "in the game", ie. your rival drug cartel don has his wife and kids actively involved in his business, they all know what they're getting into and thus are fair game when the button man knocks on the door.
2. Your rival is a coward in hiding, you use the family to draw him out. Once they show themselves you let the family go.
3. Following up on #2, if you opt to not let the family go it's because you explicitly want to hurt your rival by making him suffer as he watches his family die before him.
All of which still comes down to you wanting to inflict pain onto your rival who is still alive. If he's already dead, you go tracking down his wife or kid or parents or coworker to kill them or torture them...you might as well just grab a stranger off the street by that point.
