[ OT ] How to write about fear in a person 's mind ?

A few things to try: 
 
 1 ) Show, do n't tell. Like the other poster here mentioned, describe physical feelings and use metaphors instead of saying something more simple like `` I felt afraid'' ( unless the protagonist says something like that, in which case short sentences separated from the rest of the story can hold a ton of impact ). Use your own personal experience to craft this if possible ( i.e. revulsion, nausea, disorientation - not necessarily your own murder rampage ). Assumption will almost always be weaker, but is sometimes necessary when describing things you have never felt before. 
 
 
 2 ) When describing a scene, you can use metaphor and simile to make the reader feel ill at ease before anything even happens. For example, describing a cloudy day as dreary or whitewashed sets a different tone ( sad or numb in this case ). Be creative with this, but do n't go overboard or you risk becoming cheesy! 
 
 
 3 ) Do n't forget all of your senses either! When someone wakes up surrounded by bodies, they might feel sticky ( with blood ), or nearly vomit from the smell ( bonus points if you go for charred flesh - extra barbecue flavor! ). If you build it up without explicitly saying what is going on, you can make both the protagonist and the reader share a moment of comprehension - which can be very powerful! 
 
 
 4 ) Contrast is your best friend. To do this well, you must ride the fine line between forshadowing properly and making the protagonist feel normal and relatable to the reader. The more innocent and familiar the other characters are, the harder their deaths will hit. Killing something that the protagonist ( and readers ) loves and having them realize exactly what they have done will almost certainly cause a response in your audience if you do it well ( George R. R. Martin loves this ). The subject does n't need to be human either - animals are naturally perceived as purely innocent from the get go, and their deaths often seem the most tragic! ( side note: oh my god did I just suggest you kill a puppy? Well you asked for something dark! ). 
 
 
 
 So in summary: Start innocent and friendly, with light foreshadowing and relatable and `` good'' characters, then kick the audience in the nuts with powerful imagery and the destruction of the things they love most! Oh, one last thing... I recommend that you only have ONE climax where the protagonist realizes that they are the killer, and maybe use previous murders as background info or foreshadowing. Otherwise, you run the risk of losing power at the climax!
