He has a rifle, a SMG, enough ammo, several grenades. He wears scavenged military armor. He stalks you at the edge of your reality bubble, keeping distance when you approach.
At night, you fall asleep. He comes and shoots you.
That sounds like another flavor of NPC behavior that would only be implemented once NPCs currently gain better AI, the ability to act autonomously and function more like other players, not get themselves killed 5 seconds after asking you to find an inhaler, etc.
And of course an NPC like that would only show up in end-game yeah? When the player has had time to prepare for ambushes with either their own NPCs taking the night watch or the alarm system CBM. I mean getting murdered like that without fair warning might be “realistic”, but it would be far from fun.