Inequality of outcomes is not only inevitable but desirable. It's a natural result of people being left free to pursue their ends, and it makes trade possible. I absolutely want others to have things I don't, because that means they have things I can trade for. Inequality is a problem to the extent it results from corruption and rent-seeking. People should not be allowed to gain wealth by rigging the system. But the billionaires you're referring to generally grew rich through entrepreneurial success, and would've done as well or better on a pure free market. Besides, philanthropy is great but it doesn't benefit the world nearly as much as innovation that spurs economic growth.