Sure, go ahead and use your "free will" to disobey God. Let me know how long you'll like being in the belly of a fish. The record is three days.
You are either a slave to sin or a slave to righteousness.
But, "free will" is a curious thing. If you had it why wouldn't you use it 100% of the time to be obedient to God?
The very simple answer is pride. All humans have pride, honest pride, and vain pride.
It is the vain pride that calls us to sin, just as Adam and Eve did.
Why does a child not listen when their parents tell them not to touch the hot stove? Pride.
Why do people say things like “I’ll show you!”? Pride.
Why do people not obey God? Pride.
Nobody said it was logical. Just as, why don’t people trust God, a perfect being? It happens every minute of everyday.
Of course, God chastises the sinner. How else will the sinner reform his life?
People struggle with sin every moment of the day. It is part of our fallen nature, and our imperfection.
Fortunately, we have recourse to God and His mercy when we do fall, and His grace to help us battle sin, as well as strengthen us.