I'll admit that I'm skating on thin ice here, but my point is that you can't prove that free will exists until you do something that's opposite to what God wants; namely, a sin must be committed. So since the existence of free will inherently means that sin exists, free will must be wrong.

And I'll admit that Calvinists kinda-sorta believe in free will to an extent, but not really. The rules are certainly remarkably complicated and seem to exist to make it more palatable to the population.