I'm going to share this little piece of thought with you as well, since I commented it in a Sketch post…
Dream has come to see his purpose as something almost sacred to make others feel better, to soften the world simply by existing, as the embodiment of positivity, he believes it is his responsibility to bring light wherever he goes, even if that means setting his own emotions aside not even giving himself the opportunity to understand them. He convinces himself that this self erasure is part of his role, that it is what he is meant to do, is his role to fulfill.
Because of this, Dream struggles to understand that not all pain can be traced back to Nightmare. He doesn’t realize that he, too, is allowed to feel things that aren’t bright or comforting, being “positive” does not mean being untouched by darkness, humanity, but this is a truth he only begins to grasp through Killer.
To Dream, Killer is redeemable because he sees him as a PRODUCT of Nightmare’s influence, as someone shaped and distorted by him, just like what he did to Passive. In his mind, if that influence were removed, everything would fall neatly back into place, yet emotions are not that simple YK, and neither are people, but Dream is very much locked into his black and white mindset, purely good or bad.
It is through Killer, THROUGH LOVING SOMEONE WHO EXISTS ON THOSE GRAYS, that Dream starts to learn this. That positivity does not mean denial, and that caring for someone does not require sacrificing every part of yourself, in learning to see Killer as more as person than a rol, Dream slowly learns to see himself that way too, allowing himself to be more… human