If it was so planned, and they came to specifically kill two almost 90 year olds, why be so dramatic - why not just shoot them? It seems like rage to me - and group hug back to all. Can you tell this is the last thing on my mind before bedtime? I hope it's solved soon and I don't have to read the book years from now.
You and I seem to be thinking along a similar track Happyshoes. I think it was one or two young people known to the Dermonds. One of them with a lot of anger towards Mr. Dermond for some perceived insult or perhaps he'd been turned down by Mr. D. when asked to "borrow" money or something of value. However, imo, this was meticulously planned by amateurs. Had it been a burglary gone wrong, I think more forensic evidence would have been left behind.