Wait wait wait wait.
From what I read here, you say that the Garden of Eden, Eve, the story about the snake- all that is true?
....but the snake wasn't Satan?
.....or are the whole story and the characters in it also fictions as allegories?
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Sin is a religious term. Sin was created by religion to make people feel guilty, then they can contribute to religion monetarily or otherwise to appease their guilt.
People do wrong/bad things for self interest or other strange reasons. But it's only sin when religion gets involved.