Although it appears to be a simple concept at its face, original sin brings aspects of varied origins into play. It isn't enough to simply say the woman ate the fruit, gave some to the man, and thus was original sin born.
In the Garden, four free wills were in action: both human's, Satan's and God's. Only God's free will was (and is) immutable, eternal and perfect. The human free wills were not tainted by any failure, and therefore, having been created without sin by God's immediate acts, their point of contact with God was necessarily His love. They were worthy of His personal love, as there existed affinity between the three.
Man's free will was subject to change which could lead to imperfection, but the only way this would have been possible would have been to eat the forbidden fruit. However, the weakness of love as a point of contact with man is found in human volition. God's love could not provide eternal security for man because of that "spoiler." This is the tip of a much bigger iceberg, but it warrants being made.
The woman's vanity in considering the fruit was not a sin. The man's thoughts, relative to assessing his wife's situation and deciding to join her, were not a sin, either. That bears some consideration, in light of the many teachings of the Lord Jesus Christ, relative to our thought life. The only sin possible for either of them was to break ranks with perfection and join Satan's policy toward life.
In the Garden, there were two trees associated with the business of their spiritual life. One was the tree of lives (not, as erroneously translated in the KJV, the tree of life), and the tree of the knowledge of good and evil. The first tree represented Adam and the woman's positive volition toward the plan of God. As long as they continued to follow God's plan, they enjoyed the fruit from the tree of lives.
The second tree represented Satan's plan. This tree is not the tree of the knowledge of good or evil, but good and evil, linking similar concepts. "Good and evil" is the sum total of Satan's policy for ruling the world and for proving himself to be as good as God. Eating from this tree was to acquiesce to Satan's policies and plans.
When we understand Satan's reason for revolting against his own Creator and recognize his current objectives, we have a frame of reference for understanding evil. He promotes human good and good deeds apart from the filling of the Holy Spirit and doctrine, or any solutions contrary to the laws of divine establishment.