Until now, leaving the other martial artists aside, the deaths of a first-grade martial artist such as the southern ghost emperor and three second-class martial artists like Black and White Impermanence and the King of Equality alone have caused a heavy loss for the netherworld, losing several of their top tier powers.
More importantly, Everlasting Observance, the netherworld's stronghold that has been operating in Tiandu for a long time, has also been completely uprooted. That is a significant loss.
All these have affected the netherworld's reputation as news about them spread.
For any assassin organization, once doubts arise about their ability to complete tasks, their business naturally suffers, and this loss of benefits is what matters most.
That's why the southern ghost emperor was determined to kill Pei Xuanjing and restore their reputation. This was something they could not tolerate.