Jesus Christ is both justice and mercy. The Law of Moses required for the woman to be stoned; He didn't fail to invite others to stone her. He even opened up the opportunity for others to condemn the woman, but only if they met the qualification that He knew no one (besides His mother & Himself ) could meet: 'let the one among you who is without sin be the first to throw a stone at her' (John 8:7).
Among all the people that were present, Christ could've immediately condemned her by His own standards. But He did not condemn her; He showed her mercy.