The fact that anyone can amass that amount of wealth means that far too many people were not paid enough.
How about this: Tax them as much as it would take for them to experience hardship, just like the rest of us.
A family making under $75,000 these days is barely able to make ends met, so the tax they experience causes suffering. Yet the way we tax billionaires makes no difference at all to their quality of life. Tax them more. Until they feel it.