John 8 begins with a textual controversy. It is the story of the woman caught in adultery. According to Biblical scholars, this story does not appear in earliest manuscripts of John (or it appears elsewhere in the book and some ancient texts have this story in Luke 21). With these textual difficulties there are a variety of responses. The most likely option is that this is an authentic story of Jesus ministry (it certainly fits the character and flavor of the rest of the Gospel stories) that circulated independently of the Gospel and was attached to John (or Luke) for its preservation.
The story itself presents an interesting dilemma: according to the Old Testament law, if a couple is caught in the act of adultery BOTH parties are to be stoned to death (see Leviticus 20:10). So the question rises: where is the man? If the Pharisees are actually interested in keeping the law . . . there should be two people in front of Jesus. Jesus response is let the one without sin cast the first stone. This is consistent with Jesus saying "don't take the speck out of your brother/sister's eye when you have a log in your own (Matthew 7:3-5)." And, "Judge not and you will not be judged (Matthew 7:1)." Why do we condemn the behaviors of others and so seldom condemn our own behaviors.
I once preached a series of sermons titled "Jesus said: 'I am.'" As we seek to understand our Lord and Savior, notice the times he identifies himself with the words "I am". The name for God that is given in Exodus 3 (at the burning bush with Moses) when translated from the Hebrew translates into: "I am who I am". Jesus, in using this phrasing is identifying himself in the same way. He says "I am the door . . . the way, truth, life . . . here in John 8:12 "I am the light of the world". And, the one that really riles up the religious folks, "before Abraham was, I am (8:58)." In the 8:58 reference Jesus claims to be precede Abraham. Remember that each story in John comes to the same conclusion: Jesus is the Son of God the Savior of the world.