How would I go about expressing that he's been traumatized? At the beginning of my story he's this 21 year old kid who's a huge jerk. Loves to drink and loves material things. By the end of the story his best friend and brother are both dead, he's been tortured and he was forced to commit murders.
You would describe it with visions of his past memories, regret of their deaths, and the feeling of no happiness where he doesn't feel the same.
By describing how it has changed his outlook on life. What does he think when he goes out of bed, looks himself in the mirror, or thinks about the future? What does he think when he sees a murder on TV?
By the end of the story he's going to be a ghost of the person he used to be. I was also thinking that because he's psychic he starts seeing the ghosts of the people he killed.
Possibly, but I wouldn't go overboard with the ghosts thing. That would mean that either he is actually seeing ghosts in which case there should be some sort of mystical reason for that or he's mad. It is both very difficult to write a coherent story like a madman (particularly in first person) and it makes it harder for the reader to take the character seriously, which you need for a story.
I wouldn't do the ghost thing. Maybe you could have him walk down a crowded street and in a blink of an eye, he sees someone that looks like his brother or one of his victims. His heart stops for a moment, thinking "Could it be...?" but then the person turns around and he realizes it's not what he thought it was.
Show not tell. Remember that. PTSD symptoms - nightmares, general feeling of fear (in real life, this means things like not wanting to turn off your lights at night and/or sit/lie in places where you don't have a full view of the room around you), forgetfulness (you can show this by having him do the same thing repeatedly, like lock the door), lack of focus, etc. Also, he'd be upset by, and avoid, things that remind him of the trauma. These things could be obvious or not, and not all things that would seem obvious would affect the person.
You don't have to reveal all at once. He's a jerk, he drinks and he is materialistic. Thus, his journey starts here. Now the work is to reveal little by little why this character should gain sympathy from your readers. Peel back the petals little by little. Maybe he's in a bar drinking with a friend and the friend brings up something that triggers an angry reaction, make it mysterious. Further into the story, when this characters has your readers intrigued, you can reveal some more about his past. By the end, the reader is rooting for him, then the devastating blow, he'd been tortured, forced to murder...now he is fully redeemable in the eyes of your readers. Something like that would work for me.
Another idea I had was that he's left in this catatonic state and the story ends in the pov of his dead brother's romantic interest. That's probably a stupid idea though.
