Regardless of what else they did, they hoarded enough wealth to live hundreds of lifetime without ever wanting for anything while there are people on this planet who are starving, homeless, or can't afford basic medical care.

Being a billionaire itself is unethical. They are excessively greedy to the point of evil. If they weren't they would have stopped hoarding wealth hundreds of millions of dollars sooner.

If I had enough food to feed a hundred thousand people for the rest of their lives, more than anyone could ever eat in thousands of lifetimes, and I kept hoarding more and more food while people were starving you would ask what the fuck is wrong with me, there's no way I'll ever need all that food. I would rightly be called obsessive, greedy, and a sociopath. Add one layer of abstraction and we hold these monsters up as heroes.