> Even chimpanzees would probably understand and disagree with you
I think it's important to stress the practical side of the argument too, rather than just appealing to intuitions of fairness.
Bill Gates probably believes (and is probably correct) that for any sum of money he is willing to part with, he could do more good by directing it toward carefully selected philanthropic causes than by donating it to the government. So when acting alone, there's no reason for him to unilaterally pay extra tax, unless he believes it will have very good indirect effects and/or is just seeking a PR boost.
He also believes that if people in his class (including him) paid much more in taxes than they currently do, the world would be a better place than it is now.
These two beliefs are entirely consistent, for at least two reasons:
* For a lot of people in his class, the likely alternatives are not philanthropy vs. taxes, but non-altruistic spending vs. taxes.
* There are, at least arguably, some things that government is better suited to doing than private charities are. Perhaps these tend to be the kind of large-scale projects that could be funded by higher taxes on all super-rich people, rather than anything that would result from a donation to the government by a single billionaire.
Even if your opinion of government is very low and you disagree with the second point, the first should be enough to show that his position is perfectly consistent, and would be even for someone unrealistically altruistic and willing to be on the wrong end of an unfair situation.