The members of the society of thieves are the leaders of very gang. They were in a meeting considering what to do to Arthur when they were attacked.
Those two people said that Arthur thought nothing of them to the thieves and thugs that were still alive after the two of them raided their meeting. So not only did Arthur kill many of them, he also humiliated the ones he didn't kill.
It was after this event that Arthur had a public fight with Master Sky Claw. Some of these criminals were in the crowd, so they witnessed why Arthur wasn't afraid of them.
Arthur was strong, but they were still not willing to give up on revenge. Unfortunately, going after him would only mean losing their lives in vain.
But things got worse. Arthur continued to push their buttons and humiliate them by wiping out the gangs in the city. He disrupted their business many times and finally caused the collapse of organized crime in the city.