My understanding is that Jesus found a way to show mercy to the woman taken in adultery whilst at the same time upholding the Law.
Everything Jesus did EXPLAINED the true intent of the Law, rather than contradicting it.
So He was acting within the requirement of the Law when He said, "Neither do I condemn you - go and sin no more" - because the Law required eyewitnesses; and the eyewitness was to be the first to cast a stone at her. Not a single person remained to condemn her. So due to the absence of witnesses, Jesus was not required by the Law to condemn her.
It's a mistake to interpret the text to mean that Jesus rode rough-shod over the righteousness of the Law in the name of grace. Jesus found a way to allow mercy to triumph over judgment without actually changing the meaning of justice.
Jesus was not one of those whose acts of mercy inadvertantly promoted licensciousness. In fact, instead of diluting the meaning of justice, it so happened that as a result of the way Jesus handled the situation that day, all those men went home with a far stronger personal conviction about the righteousness of the Law than they'd ever felt before - and yet it was mercy, not judgment, that achieved this.
Man's wisdom errs either on the side of lacking mercy, or on the side of lawlessness. But God's heart finds a way to let mercy triumph over judgment without diluting righteousness. That's the Gospel!