Do you think Nightmare would be petty if Killer gets a happy ending?
I think it depends a lot on how you want to characterize Nightmare, but I do think he would be very resentful for quite some time if his relationship with Killer is prolonged. Or maybe not. Because no one can stop them from finding another Killer and repeating the abuse cycle. An interesting change would be if Nightmare stopped replacing Killer, for whatever reasons you want, and instead dealt with the situation of his freedom. If they ever deal with it. For me, Nightmare runs from the consequences all the time; he learned not to fear them because he convinced himself that what he does is the way. Many of the people he surrounds himself with are products of his own bad decisions (Killer, Dust, Horror, Error, Ink), yet he doesn't seem interested in learning from it. He's smart, but he's still a bastard. I couldn't say whether the trauma conditioned him, or if it was an external factor; I would say it's both, although I'm already talking about my own version of the character.
BUT BACK TO YOUR QUESTION I don't think Nightmare wants to see Killer again if he's free; there could be many reasons. You might think he's been hurt in some way, you might think he's doing something minimally good for him, or maybe he's just not interested!
If I were to write a story like this, I'd go with the second option, although even that's an oversimplification because relationships are much more complex. Well, canon is one thing, what I write is another :D In any case, I like these scenarios, mainly because they're a bit different from what the fandom is used to, which I'm getting a little tired of.
Personally, I prefer to delve deeper into their relationship than just "Boss" and "right-hand man," which isn't really the case because Killer didn't choose this (although I don't hate the idea of him doing this either, but that doesn't erase how badly the poor guy suffers!! And I want to make that clear because people oversimplify things and don't look at the bigger picture. I don't want the stupid waffles vs. pancakes dilemma). Mainly because, starting from their basic concepts, they're a brilliant contrast (an empath and someone who can't feel emotions—it's brilliant from any angle).
I'm not saying the fandom doesn't offer good ideas about them (signanothername, for example, has one of my favorite versions, and my goat Smakonca does too; both are juicy characterizations with a lot of work behind them, and I LOVE THEM), but my view of these characters has matured a lot in the last four years, and I'm more interested in explorations with more nuance, more subtext. I'm fed up with the horny guy and his so-called sexy god. I'm not saying it's bad, lol, but it seems kind of outdated to me. Especially when you have people with great ideas about these losers. They're definitely my favorites.
Anyways this reply became long as hell, sorry, i started yapping! Thank you for the asks <3