How could sin enter the world by the first man created if sin didn't pre-exist man?
God created
Eden and all that exists in six days. And when he had finished he judged it very good.
In the center of Eden God planted the tree of the knowledge of good and evil. Its fruit, as we're told by God later, once consumed would make the consumer like into God. With the knowledge of good and evil.
The dual aspects of reality. Good, evil, dark,light.
Then God told the man, who did not know the difference between good and evil, right and wrong, to not eat of that tree.
Then, God let the serpent enter the garden. And the serpent knew of the forbidden tree and the conditions surrounding it, which is how he knew the exact tree to slither onto so to tempt Eve. When God had only directed Adam in the rules against eating its fruit, leaving it to Adam to inform Eve.
God sets his steps.
Eden wasn't paradise when there was 1 tree planted and designed by God to initiate the fall of man.
If man wasn't to fall, the tree would not exist. The serpent would not have gained access to that precise tree. And being God created all things, the serpent could not hide its true nature from Omniscience.
We also know the fall was planned because God tells us, before the foundation of the world, its creation, he wrote the names of his elect, those whom he would call unto his Salvation, before the fall took place, in his book of life. The Lamb's Book of Life.
He chose whom he would save, and by name, before he created anything at all.