It is hard to simulate a psychological aspect into a game. Flashing Images of faces is also kind of cheap. But as I understand it psychological problems have most times interaction with physical symptoms, like headache, hearing loss, etc. These physical symptoms are way more easy to implement. Also I thought about a more indirect approach, e.g. change the humanity logo (the evil on) into a sad looking face, to try to let the player feel some guilt for his own actions.

-1 night is fine.

It is hard to fix a social problem with game mechanics, if it is even a problem. Also as this thread goes around and around just my ideas to come by this problem. 1. Change the humanity logo face to a sad looking one, if the humanity drops. 2. Remove the debug window. (shows kills etc) 3. Remove all kill messages. 4. Chance of damaged loot form players.