I’ve been thinking about ways Killer might’ve been instrumental in furthering Nightmare’s goals of widespread negativity, and I think he probably encouraged Nightmare to engage a lot more with mortals in Universes that aren’t aware of the Multiverse as a whole.
Universes that don’t know either Killer’s or Nightmare’s reputations or faces, but also don’t know anything about the Star Sanses, or Dream or Ink.
Reminding Nightmare that human mortals have their uses, and their souls are often stronger than most of monsterkind, reminding the Boss that cases like himself, Dream, Ink, Error, and even Killer are basically outliers. Not typically considered the norm.
I think Killer probably came up with the idea of creating cells inside those universes, secretly training and manipulating both human and monster agents to serve Nightmare’s goals, regardless if any of them are fully aware of that or not. Targeting any dumb or desperate fool, especially those who have any political power or authority over a certain place like a country or an army.
Encouraging wars, gang wars, revolts, territory spats, and acts of mass terror in these universes that could give Nightmare all the negativity he needs without anyone being all the wiser.
With everyone being too busy pointing guns and fingers at eachother to notice that they’re being used for something bigger. And Killer will keep the guns of war going by discretely turning everyone against their own groups, spreading rumors and propaganda.
Assuming Killer targets people who aren’t exactly important to that universe’s script, he may even be able to evade Ink’s attention for some time.
So while the Star Sanses might think that Nightmare only has himself, Killer, and possibly also Horror, Dust, and Cross/XChara doing his bidding.
He actually secretly has both deals & agreements with certain Multiverse-Aware universes—supply Nightmare and his Gang with resources & they’ll leave you alone—but thanks to Killer’s idea, he also has billions of regular Joes working for him and they aren’t even aware of it.
And, of course, Killer would be the one out recruiting people. And I think that the information about the operative cells would likely stay classified strictly between Nightmare and Killer, even if Horror and Dust are very likely suspicious.
But if any of them try to confront Killer about it he of course plays the fool, acting loud and annoying so all they’ll want by the end of the questioning is for him to just shut up and go away and they’ll completely forget what they were even asking in the first place.
(Not Dust though. Cuz while I get the vibes that Horror and Cross just wouldn’t want to deal with Killer, I think the similarities Dust sees between Killer and the anomaly only prompts Dust to pay closer attention to Killer’s actions and less his words.)
I think Nightmare’s Gang can be pretty cool if the rest of the Multiverse sees them as mostly a group of ragtag criminals causing mayhem and negativity wherever they go, but it’s just a front for a more numbered, highly structured and functioning organization with bigger, long term plans.
You could walk past a member of Nightmare’s organization and you wouldn’t even know it. The member themself may not even be fully aware of what they involved themself in.