Well sure, stealing from others is a violation of the NAP. That's part of the "keeping the peace" duty I mentioned in my above position that the state should keep the peace, uphold property rights, and enforce contracts.
What's the NAP? In any case, we rely on the state to uphold those rights, and since we cannot expect the state to work for free, we accept a percentage is needed for the maintenance of those right.
Sorry, non-aggression principle. Basically, no using force against someone else until he does it first. You're right that we do have to have some government to uphold basic duties. However, those duties are pure public goods and should be billed as such. Sum the total expenditures each year, divide by the number of adult citizens, and directly bill each citizen that amount.
I think it should be normalized to the amount of wealth rather than pax, because a larger pile of wealth relies more on this authority to maintain it. I'd even think perhaps supralinearly so.