You're right, it doesn't make a lot of sense to have revenge against someone you owe money. Perhaps the butcher frequently harassed the father about it, or aired grievances in public or something else that made it worthy of revenge.

As a side note, I'd use lent instead of borrowed, the butcher lent the father money. Borrowed can be used in this way, but at least in my experience typically is only used as the father borrowed the money (from the butcher). Regional variations exist though, some of my friends from other areas of the US do use borrowed in both ways.