The only definition of legal comes from the government, which requires taxes to create and enforce laws. So "taxes are legal theft" is inherently self-contradictory.
Also I wrote this a while back as to why taxes are justified: https://steemit.com/anarchy/@telos/the-landlord-analogy
I'm talking about income tax specifically, and the contradiction is kinda the point... They say it's legal so they can do it.
Taxes are unlawful because they harm people by stealing their income. Mind control which is the etymology of government is slavery. Governments are fictions and don't actually exist. The truth is that they are just men and women stealing from people. Taxes cannot be morally justified. Robbing peter to pay paul is still theft and it doesn't matter how you dress it up.
The supreme court has on numerous occasions straight out said that government has no duty to the public, which means the public has no duty to the government. In a very real way this is an admission of fraud and theft. Sorry as I didn't mean to make you feel bad.
You can make many claims about taxes. "Unlawful" is not one of them, as they are literally the written laws.