Drug addiction can happen among rich families, yes. Coke is common among them because it tends to be an expensive party drug.

As Noel_Skum mentioned, the dealer likely wouldn’t kill the guy who owed them. Some much more likely ways the dealer would act:

• Kidnap the debtor and ask for a ransom from the family
• Get info on the family in order to kidnap some relative
• Steal from the family home (likely convincing the debtor to do that, as they likely know where mom’s jewels are, etc)
• The two make a plot to kill the parents so the child can inherit the entire fortune and pay off the debt.

All of these have happened in real life