It is also true that the Four Directions Sect sometimes slaughters everyone in the city that they control. But that's only when they are about to lose control of the city and don't want the Martial Body Sect to steal all of the people in the city.
They kill everyone in the city just to make sure that they reincarnate in the Central Domain, not because they want to eat these helpless mortals. And if they had a choice, they wouldn't kill the mortals. They only did it because they had to.
So if this city were to fall under the control of the Four Directions Sect, Loki wouldn't have to kill the people after recruiting them. But he is willing to go through so much trouble to secure semi-permanent control of the city. His goal is to dine and dash.
He wasn't the only one doing this. 7 other people in his group were also roaming the city, generously helping people join the Four Directions Sect and killing them afterwards.
