Underverse and the Undertale Multiverse
I don't know much about OuterKiller. I only became a fan of Undertale after the first season of Underverse was already out. But it seems like by having Killer kill Outer, it removed a possibility for connection-ripped a giant hole through Killer's capacity for connection, and in doing so, tied him even inextricably tighter.
I don't like Nightmare. I don't like Dream very much, but that's more to do with wasted potential than the evil, the harm at Nightmare's core. I don't particularly like Killer-not in Stage 3, at least-but by having Killer around just to murder a couple of people, it knocked the breath out of any hope that Killer could exist without killing people.
And the thing is-Killer doesn't want to kill people. He wanted to see something new, he wanted to get out, he had to be forced, timeloop after timeloop into murder, and he doesn't like it even then. If he had the opportunity to get out, he'd take it. If he had the opporunity to reset the world, he'd take it. If he had the opportunity to reset his world and then leave, no one from his original timeline would ever see him again, because he'd be ashamed. He'd love Outertale-he'd love hanging out with a guy he's never met before, never had to kill, and learning about the stars. None of the Murder Time Trio have more murder as their life's wish.
Dust snapped, and killed everyone. The thing he wants most is for everyone to be safe, everyone out of the eternal murder cycle, and if he can't have the former, he'll be satisfied with the latter. (Not satisfied, really, but what does it matter?) He doesn't want to keep killing people-the Multiverse has so many more posibiltes than an endless, undying cycle of death.
Horror-yeah, Horror wants to kill people. Humans specifically, but he did doom the Underground to an inevitable slow death, which he keeps maintaining under a threat of fast, violent death (Breaking the barrier as monster cannibals isn't going to go well for anyone.) But he loves his brother. Not enough to sacrifice himself for his brother's sake, along with the rest of the Underground-not enough to save him-but he plays along with Papyrus's stupid games with a minimum of fussing. The reason that Horror's killing people is that there isn't anything left but that, or starving, or dying. He's got restrictions, limits he won't cross. He hasn't killed his whole underground (Cross didn't either, just, like, his eight closest family members and friends (XGaster is not included in that, in any way) and then destroyed his universe on accident, because he didn't listen to the kids trying to kill him when it came time for doomsaying. Understandable, if unfortunate.) He doesn't kill monsters, mostly. I doubt there's anything you could do to make him kill his brother. Set him loose upon the multiverse, and he might bring home a dead human, but he's much more likely to get food for his brother (Snowdin is a distant second) and maybe abscond into the Multiverse with his brother. If he had a stable life, and killing humans would destablize it, he wouldn't kill humans. Simple as that.
They're the Murder Time Trio for their unusual willingness to kill, but they're all Sans at their core. One way or the other, and killing people isn't their greatest desire. They deserve to escape Nightmare, to exist outside of Nightmare's reach. (If you like Dream and Nightmare, you might like Celestia and Luna from My Little Pony. Same story, the first two episodes, but with a happier ending.
