WELL whillieH
you might say that Because EVERYTHING
(the Earth, Animals, Etc) was
put under
ADAM'S dominion, that when
SIN entered
ADAM everything UNDER him
suffered the same.

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(Warning: Written in long-hand for explicit detail)
The problem is not so much about sin, and the entrance of it perhaps, so much as it is about separation. They disobeyed, and sundered the connection between God and man, not because God sundered it first, but because mankind - Adam and Eve,
hid themselves,
they fled. They made the first move in separating themselves from God. It was God who sought them first...only to find (alas to his knowing already) that they refused to repent, and in that repentance turn back to him. It is only when we run away from God, that we miss him, and inevitably fall into sin. Whether or not it is running away from God who is Love into the arms of The Law which is Death, or else running away from God into the arms of pagan devils, or even into ourselves, and our own sinfulness.
Sin only occurs due to a lack of God's presence: If we walk in the spirit, we won't fulfill the desires of the flesh.
It is from Death that sin comes; Without The Law we would not know of sin - The Law is Death, the wages of disobedience/sin is The Law, which is death.
When Adam disobeyed, dying he died. From that dying he began to sin - just as a rotting corpse begins to stink.
Sin did not enter into Adam - Death did! Because he, and Eve forfeited life by running away from The Life.
When Adam and Eve ate the fruit, they disobeyed God, and realised immediately the consequences - They became aware! They had a revelation of their shame, their nakedness, they lost their innocence which rendered them responsible to The Law, which is death, because The Law is the mirror by which we see our shame, our nakedness, our destruction, but it does not clothe and heal; The Law is thus - The wages of sin, is death, that is why Grace, which is the superseding force above The Law is a gift, which is Life. The Law is "wages earned", Grace is "a given gift undeserved".
Adam and Eve, upon taking the first bite where already dying, and dead - being responsible to The Law and its wages, the wages of sin - Death. It is only by running away from God, and refusing to repent and turn back to Life that they could not enjoy the fruit of the tree of Life - the gift of life; and so died, and death became the enemy, the
real enemy of man.
That is why Death is the "final boss". Satan is a created being, but death is a form of absence; a void...a thing beyond a created being that cannot be stopped except one who is beyond creation - God, who is Life which like light in darkness fills it, and replaces it, casting it out, overwhelming it for as long as even a spark of it remains.
I just wanted to remind everyone that Sin isn't the thing that entered into Adam, and into the creation subjected to him - it was Death, caused by the unrepented separation from God when they ran.