I know ghouls as a general rule are souless, but in this specific setting they never mentioned them lacking the soul they simply state they are "creatures who lost their place to return to" and that they keep being made because the world tree isn't working and they established that the world tree helped with recycling souls and passing on the Dead, they also formed a flesh golem that was made from unrested corpses but is made stronger by resentment, to have said resentment you have to have some sort of soul or ego in there. Given these facts the cause of ghouls in this world can be stated as a person dies horribly, their soul can't rest, attempts to go back to their body but their body is dead so they can't properly resurrect thus they become a ghoul. Hell the soulkiller sword makes perfect sense too since he's able to actually properly kill them using said sword when it was shown other people using weapons could damage them but they would just ignore it.(Like in the previous chapter where they reveal how the ritual works and he stabbed it through the heart and the other guys cut off its arms but they just sort of put it back in the box. Or how they dispose of them by just tossing them in the sewers since it seems to be incredibly hard to kill ghouls.