Rules wise, there is no list of punishments for crimes. Each village, city, and hovel has its own law, and the DM is the one who decides how the world reacts to the players. So, it falls to the DM to decide what the law of the land is, and how people will react to being assaulted on the street. In a lawless land, there is no justice. In a big city, I think jail time. This can lead to an interesting challenge for the party to break their friend out of jail.
Now, if there was a contextual reason why this player chose to attack an innocent man, or if you guys decided as a group that doing whatever you want is fair game, you can proceed with an in game response that you feel is appropriate to the city.
However, if this player is just being disruptive for the sake of it, and his actions fly in the face of what you guys determined you wanted out of the game, then a better response would be to talk to him about why he is being so disruptive and how you can make the game more interesting. Then, let him off the hook with a slap on the wrist.
You say he is going off the rails, which leads me to believe it is the latter. Always remember that you describe what happens in the world. If your player is being disruptive, it's your right as the DM to say "Why don't we take a break for a quick second before I resolve that," and then take a second to talk to him in private about what you both expect from the game.