In PnP basically the game director takes your character sheet away from you if you start to accumulate that type of behaviour, ask you to roll a new character and if he/she feels a little bit sadistic your 1st quest with the new character implies either you killing your old character,or your old character is trying to kill you. You can even lose humanity by "mistakes"( i did a shootout i killed a poor kid accidentally,i'm not a psychopath so i have nightmares at night),
In computer games,Bloodlines has both Masquerade and Humanity points that works that way... even if you are a "monster" the devs didn't allow you to unleash the beast.
This would have been a good system in cp2077 for police: you hit 0 points humanity MaxTac appears and 1 shots you.
Just add humanity as an attribute at character creation that you cannot recover when leveling, but you can lose by killing "non-mission targets"-killing a cop, in cold blood in the middle of the street not involved in a mission should be punished i think-with perhaps some quests (free helping NPCs, there are few of them) that allow to recover some. Remove spawning cops,just fixed control posts(they don't have budget anymore for patrols) without reinforcements.
This adds a mechanic that respects lore and allows player freedom with some margin (say humanity minimum 3 at character creation,10 maximum,5 "non-hostile kills cost you a point of humanity, between 15 to 50 innocents killed...i doubt i killed by accident more than five, but give some freedom).