Player characters like to fight fire with fire, and most often with a preemptive attack. Subtle violence takes that option away, puts your players and their characters on the back foot, and forces them to think more, before grabbing the dice and selecting a target.
The horror comes from playing on your players’ feelings of helplessness, whilst still leaving them unfettered player agency.
You want them to know that they are under threat, that they are being attacked, but they don’t know who by, when the next attack will come, and how. How do you defend yourself against an attack you cannot predict?
The violence can be physical, but you can also use psychological attacks or attacks against property. What if things of little monetary value, but great emotional value start to disappear.
Another one that can have a big impact is having a character wake up to find an obvious lock of hair missing. We all have ideas about what sort of witchery can be done with a lock of hair or a fingernail clipping.
Subtle violence is a vehicle for increasing anticipation and dread, ramping up the tension before hitting the characters with something bigger, and more threatening.
If your system of choice is any version of D&D, then subtle violence is often a good device. If you want to make players sweat, pick threats that rely on saving throws, not hit points of damage. Saving throws put the responsibility on the player, their characters fate is in their own hands.
These tips are taken from my Scarier Horror Play book. You can get 40% off this book at DriveThruRPG using this discount link. You can get 40% off all new releases as a loyalty discount if you join my contact list.