This is actually getting super interesting. We already saw that a spirit can get stronger by eating other spirits, in fact that seems to be pretty normal in the "ghost ecosystem." The guy wasn't killing CATS, but he seems to casually kill HUMANS, so the stuff we saw in his apartment, the blood and the suspicious bags, makes it seem pretty clear that he's out here being a vigilante. He kills other criminals and animal abusers, or at least, that seems to be the implication. If that's true, then it makes sense that his mother's ghost was so absurdly powerful. Even if she was twisted and cruel, her primary fixation was her obsession with her son, keeping him safe and under her control. That would still be true of her as a ghost, so she presumably ATE all the criminal spirits of the people he murdered, and prevented any other ghost from possessing him. Which would be why she was so powerful. Like the monster hag and the trashcan spider, she was a 'hungry ghost.'
If that's true, then getting rid of her may have improved his life in the short term, but it's going to be a big problem in the long run, because now he doesn't have a guardian monster sitting on his shoulders eating the criminal ghosts of the people he kills.
And now, of course, we finally get to see what the incomprehensible Chinese gibberish speak of the guardian spirits and the fox god means, because they're coming around for... something. Further payment? A reckoning? After your three are up we eat your soul?
I want to see the axe guy come back. I love the idea of a crazy and twisted ghost who spends all of his time hunting down the other ghosts that try to possess people, and stuffing them in a sack. Like some kind of vigilante Krampus. All the naughty ghosts get axed away.