David had a motive
No question that David had a motive. Wright pointed that out at the trial. Of course Karam wants people to believe David didn't have a motive.
My best notion is that David thought he was going to be head of the new house and then he found out his father was going to have a room. That could have been the catalyst. Was that the shattered dream , broken promise he talked about in the booklet Innocent?
At trial he said his father wasn't going to be part of the new house yet he told Binnie his father was going to have a room in it. It would be interesting to know why he said one thing at the trial and something else to Binnie.
Did he say his father wasn't going to have a room to make it look as if that could be a reason for his father committing suicide? Apparently Margaret had pencilled in a room for Robin on the plan so the police would have been aware of that.
I have always felt Wright summed it up reasonably well. when he said "It was to be his house and if he couldn't get it he would have the inheritance,the money his parents had put aside for the new house."