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Girl, I genuenly thought first that you were gonna say he's sleeping.
But on the other hand, Nightmare can take away negative emotions to use them, right (Or at least that's my headcanon, idk how you feel about that)? But he isn't taking away his negative feelings, because he enjoys it. And that could be one of the reasons why he wanted Killer to join him in the first place. He wanted to feel all that negativity stronger, he wanted to enjoy it more, so he came up with the solution that he brings the source, A.K.A. Killer closer to himself.
And so, Nightmare isn't letting Cross, Dust and Horror help him neither, because with their help his negativity source would get weaker in minutes.
i don't think we talk about when Killer isn't smirking or causing chaos.
Those dark moments he has alone in his room when he can't think of anything to do, or is too unwell to get up and do anything. Nothing can distract him from the horrors of his trauma.
Hatred having no outlet other than himself.