‘But when they continued asking him he lifted himself up and said to them, “He who is without sin among you, let him first cast a stone at her”.'
His next action is specific and underlines the words that He speaks. ‘He lifted Himself up'. He straightens up and looks round on them. We can almost see the sad yet compassionate, look in His eye as He passes His verdict, so unexpected to those who are seeking to trap Him and are awaiting their moment of triumph. He agrees that the one among them who has never broken the Law shall be permitted to carry out the sentence. If they are without sin as He is, then they have a right to do as they suggest.
To their credit Pharisees did acknowledge that they had failed to keep God's Law. They even believed that the sinfulness of Israel and their own sinfulness was why Israel was suffering and they believed, and taught, that if only the Law could be kept fully God would bless Israel. Thus they had to be wary. To pick up a stone would have been to deny their own teaching. They were caught in their own trap.
Furthermore there may be in Jesus' reply the suggestion, which He expected them to be aware of, that He was aware that some of the Pharisees themselves had dubious reputations. This may help to explain why the eldest left first. If there were one or two in that situation the remainder could hardly claim innocence as a group. They were condemned by the company they were keeping.
Had the questioners been sincere and genuine in their question, they would have received a different response, but Jesus was well aware that it was not their sense of purity but their hatred of Himself that motivated their action. He thus turns the tables on them by His reply, for none of them would dare to suggest to the crowds, or to each other, that they were without sin.