In his essay on social justice, economist Walter Williams shows us why slavery is immoral. Williams compares slavery to theft in that they both use physical force to confiscate one person's property for the benefit of another. In this sense, theft, like slavery, is also immoral. Slavery and theft are acts of aggression, and thus deemed immoral.
Taxation is the compulsory payment of a percentage of one's income for the support of government.
The key word in that definition is compulsory. If you do not pay your taxes, you will go to jail. If you decide you have committed no crime by keeping what you have earned, and refuse to go to jail, defending yourself with force, you will be killed. Taxation, like slavery, is an act of aggression. It is theft.