There is an argument that libertarians sometimes make that equates taxation to slavery. They say that your money is a representation of your time, since you need to put in time to earn it. As such, someone who takes your money is effectively taking your time, and thus making you their slave.
The problem I see with this argument is the claim that taking up any part of your time is akin to slavery. For someone to utterly dominate your life is a much worse situation than for someone to periodically demand a quantity of your time. More than a few societies have been structured in exactly that way: people paid their taxes in the form of labour or military service. Calling upon people to sacrifice some of their time, in exchange for getting to live in a good society, is a perfectly acceptable thing to do.
Thus, taxation is not immoral, because it is not akin to slavery.