That is certainly a step, but why not take it all the way as bilby has suggested? Distribute to it to the whole of society via added government services or reduced taxes on everyone. What is the moral or ethical argument against that?
Do wealthy people have many children though? I guess more than the average person probably.
At the time of Napoleon, a wealthy family was more likely to have more children survive to become adults.
The real question that must be answered before any government takes wealth from its citizen is, what does the citizen get in return? It's a fair question.
There are a few practical problems of taxation which must be addressed, before any particular tax is assessed. The first problem is determining how much it costs to collect the tax. It's silly to pay your tax collectors more than the taxes they collect. There is a cost, which means a lot of potential revenue sources are just not economically viable. This is why the poor are generally overlooked. What's the point of taking a tiny amount from multiple sources. The cost simply compounds with little return.
On the other extreme, ease of collection by itself does not justify a tax. Large estates are tempting, if only because one transaction can collect more than the accumulated taxes of thousands of less well off citizens.
So, the question remains, Why? Are you going after a rich man's estate simply because it's easy, or has this man and his estate benefited from government services at a higher rate than his poor neighbors?