Perhaps somewhat of a rarity among Darkners, Rouxls is completely neutral toward Lightners. As the 'useless' rules card, he never received much attention from the Lightners, even before the entire classroom went abandoned. So, as a result of their neglect, he never ended up developing any sort of special admiration or respect for them. While, yes, he does know that the Lightners supposedly created the Darkners, they never really did much for him personally, so he doesn't consider them to be automatically more significant than the Darkners.

However, unlike the King of Spades, he doesn't hate Lightners, either. Nothing really changed for him when the classroom was abandoned, since you can't lose something you never had in the first place. So, he never developed any particular animosity toward the Lightners over being abandoned, either. He just doesn't care.

Because of all this, he had no qualms with supporting King's anti-Lightner policies if it meant it could put him into a position of power. However, he has nothing against Lightners personally, so he was also perfectly willing to join Kris and co. once things started looking bad for King. But, because he has no default attachment to Lightners, he also had no problem with betraying the Fun Gang later in order to serve a Darkner leader who he deemed more powerful (Queen), even if that goes against how a Darkner is 'supposed' to behave.

Eventually, Rouxls might join the Fun Gang's side permanently, perhaps because he's finally realized that, despite being a bunch of teenagers, the group is actually very strong and will probably be the winning side of any conflict they're in. Or also, if he remains loyal to Queen past Chapter 2 (which those Chapter 3 previews do seem to suggest), then he'll be allied to them by extension since Queen is on their side. Regardless, though, he won't join them just because Kris and Susie are Lightners.