The scribes could not throw stones at the adulterer without taking a good look at themselves. Just because she was caught committing adultery, does not mean that they did not have any indiscretions either and Jesus taught them that. Jesus showed the scribes that they need to learn how to be compassionate about people and that they are not perfect. Everyone has sinned, and should be shown forgiveness. There is no such thing as one sin being bigger than the other.
The first time I read this story, I was thinking the author wanted to show how the scribes were trying to bait Jesus into condemning the woman and they wanted to see what he was going to do or say to her because they really did not trust his teachings. Jesus was a teacher and already had followers so the scribes were jealous of his power and tried to show what kind of man Jesus was in front of his followers. If he went along with Moses, then they would figure that he is not better than them and punish her. If he went against them and said do not stone her, then they would accuse him or allowing this type of behavior, think that he was weak and lose respect for him. Jesus, with his statement, passed the decision unto the scribes so that they could see the error of
References: Barnet, S., Burto, W., & Cain, W. E. (2007). The Woman Takin in Adultery. In Literature for Composition: Reading and Writing Arguments about Essays, Stories, Poems, and Plays (8th ed., ). Pearson Longman.