Tax is only theft if you can argue that there is some law that supersedes the government's law that they can raise taxes.
You need to learn to differentiate between the legal, the moral and the ethical realms. Taxes are not legally theft but if we assume that correctness is defined by law, then where is the reasoning behind changing laws?
Also, if government has the authority to rob and call it tax, then any of us can just band together with some guns, call ourselves government and do the same, and that is fine?
The big moral question then is: are various serious crimes, in this instance robbery (with violence), somehow okay because we legalize them in the name of government? The same applies even more so to extrajudicial homicide.