Illegal or immoral? These two concepts should not be confused with each other. Law er merely a description of what is legal and what is not. Morals are a description of that which is morally defensible behavior or action, and what is not.
Lets give the word to Rothbard:
If the State, then, is a vast engine of institutionalized crime and aggression, the "organization of the political means" to wealth, then this means that the State is a criminal organization, and that therefore its moral status is radically different from any of the just property-owners that we have been discussing in this volume. And this means that the moral status of contracts with the State, promises to it and by it, differs radically as well. It means, for example, that no one is morally required to obey the State (except insofar as the State simply affirms the right of just private property against aggression). For, as a criminal organization with all of its income and assets derived from the crime of taxation, the State cannot possess any just property.Of course this text cannot be understood correctly outside its full context, so I urge everyone to read the full context and understand it correctly!
This means that it cannot be unjust or immoral to fail to pay taxes to the State, to appropriate the property of the State (which is in the hands of aggressors), to refuse to obey State orders, or to break contracts with the State (since it cannot be unjust to break contracts with criminals). Morally, from the point of view of proper political philosophy, "stealing" from the State, for example, is removing property from criminal hands, is, in a sense, "homesteading" property, except that instead of homesteading unused land, the person is removing property from the criminal sector of society — a positive good.