Most of a billionaire's wealth is not earned legitimately. Pic related

Billionaires should not even exist in the free market

Pic related. The stock market is being used as a "magic money machine" which hacks money into existence. Without the stock market, there would be NO billionaires.
Earning money through speculation or appreciation of stock is not legitimate. Money should either be an exchange of labor for value, or value for value. Money is just the intermediary.

Speculation is like gambling, and should be shut down. The stock market as it exists should not be allowed.

If money actually represented the value you contributed to society, and nothing more, then billionaires would not exist, and mega-multi millionaires would be very rare.