Update: It looks like that exploit is being fixed tomorrow. From the v 1.02 patch notes that just came out ahead of time: "Named NPCs will no longer respawn after being killed in the Open World. This will prevent situations where players were able to kill a same named NPC over and over again." So smoke 'em if you've got 'em.
High-level players of The Division have discovered a way to make hundreds of Phoenix credits (currency used to buy high-end weapons) and collect dozens of special items in less than an hour by attacking the same NPC over and over again in quick succession. It is the closest thing to Destiny's erstwhile "loot cave" - a fruitful, if repetitive, means to get lots of goodies late in the game. And sadly, it is only useful to those who have reached level 30.
Bullet King is one of the game's named enemies who drops the special currency upon being killed. He spawns less than 100 metres from the door of the Autumn's Hope safehouse in the Times Square district in the northwest of the city. Players at Reddit found out that if you kill him, while leaving the rest of his entourage alone, then allow yourself to be killed, he will respawn and you can run back from the safehouse and do the same thing over and over, collecting huge stacks of credits in very little time. He will also drop two pieces of the purple "specialised" gear every time he is killed, effectively making him a great source of ordinary cash as well. There are also reports he occasionally drops a high-end assault rifle called 'Caduceus'.
But some players are finding he is not around. If this is the case for you there could be a few reasons. Either you haven't cleared the side missions in that district, specifically the supply drop mission to the west of him. Or you have already killed him and his whole crew in an earlier battle, meaning he won't respawn again for days.
There is a workaround however, if you really want to pursue this avenue of grinding (and take this from someone who has done it - it is a very dull way to make money). If you jump into someone else's game who has not yet defeated Bullet King and his cronies, then get them to disband the group, you will be left in their world. This allows you to travel to Autumn's Hope and scrounge credits and loot to your heart's content. Players say that to ensure the respawn, you must always leave at least one of his guards alive, so if you kill one or two by accident, it should still be okay.
Ubisoft are doubtlessly working on a fix to this, so if you want to get in on the exploit, now is the time. We'll have more guides and tips for The Division very soon.