How do state taxes relate to Zakat?
Zakat is a religious duty of Muslims, while taxes are the state’s right so that it can fulfil its responsibilities. If Zakat does not cover those responsibilities, the state may, and indeed must, levy taxes as well.
A state is not built on voluntary membership. It has responsibilities that cost money. It must keep law and order, administer justice, protect the rights of individuals and groups, and defend the country. For this it needs police, courts and an administration. That is why every state has some system of taxation.
There is no fixed formula for the right level of tax. States differ. They stand at different stages of development and take on different responsibilities. Even the same state needs different amounts at different times. So Islam does not set tax rates, but it does set out the duties of a government. The second verse above requires that responsibility be given to those worthy of it and that judgement be just.
Zakat has a religious character. It is a duty of Muslims, and the first verse above names those to whom it is due. If what it raises is enough to meet the state’s responsibilities, that is well and good. If not, Islam allows the state to levy taxes to the extent it needs to fulfil its duties. In that case taxation is not only permitted but necessary.
It would be wrong to see Zakat as Islam’s complete system of taxation. Non-Muslims also live in a state. Islam obliges the state to care for their needs and to give them justice, as the third verse above also shows. If they only received benefits and carried none of the burdens, that would not be fair. Taxes are therefore a shared responsibility of all citizens.