I almost don't want to reveal my tricks, but as a general summary the rules are
1) "Put the player in a situation that cannot be fixed.": Real world horror is derived from the fact that once an injustice is perpetrated, it can never be wholly undone. A murdered person leaves a hole in the universe. A person once tortured is stained by the pain. The wound can be healed, but the scars will remain. An innocent can recieve unwittingly and innocently the dividends of injustice, so that when it comes time to make restitution you must deprive an innocent of what they are now lawfully entitled to. Forgiveness and even repentance don't necessarily undo what has happened, and costs must still be paid. Innocents must suffer for the deeds of the guilty. Responcibility has to fall on the people least deserving of bearing such a burden sometimes because they are the least deserving. Life isn't filled with easy answers.
2) "Force the player to choose between two unthinkable outcomes.": Speaking of, there are going to be times when a person believes unquestionably in two things - say the rights of the individual and the value of life - which sometimes can contrast with each other. Thing about how the players and characters frame their world view. Beliefs like, "The innocent must always be protected.", can cause serious problems if two innocents mutually threaten each other. "Justice must be served", can cause serious problems if the victims right to justice perpetrates injustice on someone else.
3) "Hit the players at a primal gut level." - This depends a lot on the MPAA rating of your table, but there are somethings that alway provoke viseral reactions in players. Figure out where to push the player's buttons. Real life phobias are good fodder if you have a measure of player consent.
4) "Hold up the mirror." - In most games I've ran, at least one and sometimes all the PC's are monstrous individuals who have done monstrous things in the name of 'doing right'. Have the villains do the same things. Have the villains call out the players. Try to get the players to stare in to the abyss. Probably the most horrifying thing to me as a DM is how little horrified the average players are by the actions that they narrate their characters as doing - torture, theft, murder sometimes in cold blood, racism, sexism, sexual assault, betrayal of friends and colleages, treachery, colatteral damage to the innocent, standing idly by and ignoring the sufferings and danger others to protect their own hides. The number of PC's I've DMed for that I actually would like to know IRL is pitifully small.
Keep in mind though that not every player really wants to be scared. You make your game too horrifying and you'll probably start seeing opting out behavior. Also, try to avoid substituting gross for horrifying. There are probably things that don't need to be dwelt on and aren't healthy to narrate.