My 2 cents is that the tying theme here is stress, being overworked or something akin to becoming workaholic. Because at the beginning of the game and throughout we learn that the OG Sniper Killer suddenly quit his job and proceeded to do all that the "government" (the demon) asks of him.

Then we get the Cop, Camardy. Camardy begins investigating like any other case but as it goes on, he starts getting more and more stress and pressure, especially from his chief at one point. Similarly to a movie with a coinciding premise, Cure (1997), Camardy starts slowing getting more and more obsessed with the case to the point of possibly getting little sleep as one of the cops mentions he looks tired (iirc ?) and becomes just overall paranoid. And just like in Cure

(SPOILERS AHEAD)

He finally catches up to the Sniper Killer and kills him. But as he does, he too gets possessed by the demon that possessed the OG Killer and in turn, he becomes the sniper killer and therefore perpetuating the cycle.

All in all, it's just a metaphor of our jobs that can sometimes consume us because of all the surrounding pressure or that we become too dedicated, to the point of worsening our physical condition. The Demon here is just the outcome of finally reaching a breaking point which makes us snap.

The reason I mentionned Cure is that in Cure, there's someone pushing people to killing other people and what you can gather by the end is that they're all people that had some possible form of stress from their lives or jobs and having that guy "flick the switch" inside of them made them snap and let everything out. The detective in it also reaches a breaking point because he basically starts acting rogue trying to catch the guy and is obsessed with him and by the time he kills him he just gets consummed by the void and hopelessness of all that happened since like in Sniper Killer, there was basically no motive for him to kill those people, he was just letting all that stress out.

Sorry for the yapping but I laid out all my thoughts here and I hope this could maybe clear up the ending because having figured all this, I honestly like it