Taxation is just the government using economy of scale to provide services that you couldn't afford all by yourself, like roads and other kinds of infrastructure, military, schools, etc., and in your case, a basic level of medical care for everyone. But governments can and do use the taxpayers money badly. But that's not slavery.

There is a very big problem of slavery... holding people against their will in captivity, and forcing them to do your bidding (for instance; the very robust sex slave trade) in all western countries, including the US and Canada, as well as in less developed parts of the world.