@Spartanus the missing city lord (old guy who ate the little girl's soul and got killed a few chapters ago) is the ancestor/patriarch of the family being attacked. The city lord also used to be a disciple of the Soul Controlling Sect that the MC is from, but presumably left and went renegade to gain power and ended up as city lord.
The Soul Controlling Sect can't/won't allow traitors. Now that the Senior Sister disciple knows the dead city lord was a traitor, the disciples need to finish the punishment and exterminate his family.
A lot of the motivations are due to the cultural norms of prestige and loyalty in ancient China that xianxia settings are heavily influenced by.
Heresy is a better execution of the genre than most, but some of that execution is not spelling everything out in detail - which assumes the reader has pre-existing context to work with. Like how western-based fantasy uses kings and knights and wizards with the expectations that readers have at least a basic idea of what those concepts generally are.