According to Genesis, mankind was created perfect in every way, even in his thoughts. But humans were endowed with the ability to desire. Misdirected desire can cause us to sin.
Adam and Eve had no understanding of evil. It was the unknown and the unexperienced and this made it mysterious and intriguing. Satan, in the form of a serpent, dangled the knowledge of evil before them.
Satan also introduced the notion of being “like God.” Remember that Adam and Eve were created in the image of God. They may never have thought of themselves as anything but “like God.” However Satan, the serpent, twisted the truth to his advantage.
Satan made them consider for the first time the possibility that they were ignorant and incomplete. He insinuated that God was holding out on them, that they were missing something.
He also made himself look like an authority. He implied that he had something more to offer than God when in reality all he was doing was subtracting from all that was God. He was introducing deficiency.
The serpent deceived Adam and Eve. He had nothing to offer that God had not already given. All he could offer was the experience of something less than godliness. Their sudden self-doubt and their doubt of God made them vulnerable. They had no experience with recognizing or resisting temptation. Their desire became overwhelming and they decided to accept Satan’s lies in place of God. That was the evil. It wasn’t the apple.