He is stealing mostly from investors, Softbank mostly. Some employees who started early and were promised stock will also lose out but that's the cost of taking risks and working in a startup. That's the next best thing you can do if you cannot come up with an idea. Just like investors lost their money, so did they. On the flip side there are many examples where employees benefited handsomely because they took risks like these. Chamath Palihapitiya is a great example of this who worked at Facebook, left it early and became a billionaire at age 30 due to some other investments as well.
Now if this company goes through bankruptcy, Neumann isn't going to be able to keep this money and steal from his employees as you state. It will have to be returned in order to pay back the bond holders, investors etc. Even if it requires liquidating his holdings.