I will suggest you to study real cases of people who have reached and exceeded the acceptable behaviors having as motivation, the voices in their heads. We are all normal, everyday people and the phobias we have are not capable of scaring any of us. Spend a little of your time studying extreme cases of people who have hurt themselves and their social circle.

Of all the horror games I have played, almost none have been able to convey what a person experiences in his own hell. Psychological horror is much more than an distorted visuals and audio, it is a traumatic experience that cuts the victim off from reality.

There are endless testimonies of what these people see and hear and maybe it will be better for you as a creator but also for us as gamers, for you to try as much as possible to base your game on real cases of people who have gone to extremes.