it's not the rational part of your brain that going to stop you from pulling the trigger. It's a deeper part more "primitive" part of the brain that stops you from killing another person.

During and before WW 2, only 2% of the frontline soldiers actually have had shot to kill. The rest didn't because they couldn't, they where stopped by there primitive part of there brain. Those 2 % that can shoot people won't suffer from psychological damage afterwards, the rest usually will if they did killed a person.

These days army's train there soldiers in such a way, so they can pull the trigger easier, because they have "conditioned" there soldier to associate the enemy with shooting at them. (Bit like Maslov and his dog.) But these training methode won't prevent you from getting pshylogical damage, unless your one of those 2%.

So wenn you plan to use firearms to defend yourself, make sure you'r properly trained (so you will pull the trigger if you have too) and accept the fact that you will most likely suffer pshylogicaly if you do pull the trigger. Ofcorse you don't have to fire a firearm to be effective. Pumping a 12 gauge will be pretty effective in scaring unwanted people away.
Having something thats more effective than showing the gun and not as effective as shooting, will fill the gab between them and prevents you from doing thing you don't want to do, unless there is no other way. Or incase you discover that pulling the trigger is a bit harder than you have thought.