The crowd probably would have claimed that they wanted justice. Were they going to leave the guilty unpunished? But when they dropped their stones and walked away, they as much as admitted: there was a time when they were the guilty ones, and had gotten away with something, whatever it may have been. The older ones -- I think they left first, not just because they had more sins to remember or more time to regret. I think they left first because they already knew that, in the end, everybody would have to drop their stones and walk away. They'd lived enough to know that everybody had done something.
Here's the thing about justice: Sure there's an aspect of punishing the guilty -- but not for the sake of blood-sport. (Is that why not only crime but also "justice" can attract such unsavory characters?) The "justice" aspect is about treating people fairly, treating all people the same. And if everybody in the crowd had gotten mercy at some point -- they still had their respectability, which is some kind of redemption for a sinner -- well, if everybody in the crowd had received some measure of mercy, then forgiveness was closer to justice than punishment would have been. If justice is treating people evenhandedly, then that was its own kind of justice: everybody has received mercy.
I can't help but wonder how the people looked at each other. When Jesus said, "Whoever is without sin, let him cast the first stone" -- did they dare to meet each others' eyes? Were they looking around eagerly for the "hero" to step forward? What did they think when their neighbors wouldn't look them in the eyes? They came as a crowd, but we read that they left one by one ... which almost sounds like they slipped away in shame. The next day, did they understand mercy any better? Did they look each other in the eye with more compassion?
They were all ready to condemn, at first. Jesus was ready to redeem. But the only one who got to hear that from Jesus' own lips was the adulteress, because she stayed to the end: "Neither do I condemn you. Go and sin no more." I hope she did run into her former accusers in the following days, and that they asked what he had said. Because they left in shame over their own sins, they did not get to hear the words: "Neither do I condemn you. Go and sin no more." Did she get a chance to pass it along? There is more justice in mercy than I had realized.