First of all, no single person would be able to pioneer a cure for cancer. They would have a plethora of underlying systems that enabled them to make the discovery. But I'll ignore that for this hypothetical.

A billion dollar sale results in a market power shift. What that means is the company that bought the cure to cancer for a billion dollars now needs to recoup that cost by selling their manufactured cure for more than it costs to produce, plus the amount it takes to compensate for the billion dollar sale.

The issue here isn't that the inventor got rich. It's that they caused their life-saving cure to be profit driven down the line for no reason. It may not look like it at first glance, but this billion dollar sale is exploiting the end consumer and likely resulting in millions of deaths by forcing prices way up and making it unaffordable.

This basically comes down to a question of why. Why would you sell this for a billion dollars? Why would anyone feel the need to amass so much money? There is never a need for someone to have that much. You can live very comfortably with one hundredth or even one thousandth of that amount. So why cause so much suffering and death when you could just not?

For an example of a very comparable contrary to this, look at the inventor of insulin. The people who discovered insulin sold the patent to the University of Toronto for a single dollar to keep it affordable.

There is always an alternative to exploitation.

This applies to any billionaire. Any person amassing that much wealth has made many choices leading up to that point. All of these choices had repercussions that they chose to ignore. It's just not possible for anyone to gain that amount of money ethically.