Jamieson, Fausset and Brown on John 8:9

9. they ... convicted ... went out one by one ... Jesus was left alone--that is, without one of her accusers remaining; for it is added.

the woman in the midst--that is, of the remaining audience. While the trap failed to catch Him for whom it was laid, it caught those who laid it. Stunned by the unexpected home thrust, they immediately made off--which makes the impudence of those impure hypocrites in dragging such a case before the public eye the more disgusting.