It certainly WASN'T the Boutflour/Eatons.
If the 2 family's fought over possession of it that to me would be a sign they wanted to try to sell it or profit in some way. I would not care about a diary belonging to any of my cousins unless they were famous and it thus had some potential historical value. And I don't mean famous for how they died I mean such as a politician or scientist and it could demonstrate how decision were made...
The people who would truly appreciate her diary were dead. Her parents or children so they had something to remember her by. A diary might have things someone doesn't want an exspouse/exboyfriend/ex anything to see so I don't necessarily like the idea of it going to an ex.
All too often a diary will be sold for profit, I think its destruction was a good thing. It would eventually have been sold by an heir of the person who got it if not by the person who took possession. Someone would have wanted to buy it just because it would be a souvenir related to the killings or case and possibly hoping to find some dirt to publish from it. That doesn't dignify her memory so I say good job in destroying it.