David Cameron isn't responsible for the dickish behaviour of his father. The only thing relevant to our judgements on David Cameron is what David Cameron himself does, and you can have a field day on that topic.
I mean ... what exactly is the point in publishing this story? His Dad was greedy and used legal means of tax avoidance to keep more money for himself. So what? It's not relevant one iota to what we should think about David Cameron, both as a person and as a leader.