The Knight wants to make the Roaring happen, but all it has done is abduct
or attempt to abduct people that know the code to the shelter. There's two explanations for this, but maybe both could be true, or none of them.
EX 1: The shelter is where the Roaring is going to trigger. Maybe there's a machine inside the shelter that, when translated to the dark world, can kickstart the Roaring. Think of it like activating a nuke, they design that shit so one person can't trigger Armageddon. In addition to codes needed to enter the shelter, the dark world device would need codes to activate the machine. The Knight would have to force the code knowers into using the device, probably luring Kris and co. in to use them as hostages.
EX 2: The Knight actually isn't trying to make the Roaring happen, but is trying to prevent it by removing all the people that could help the heroes move forward. By preventing the prophecy from moving forward, disaster can never occur.
Both: The Knight doesn't want the Roaring to happen, and is trying to take anyone that may know the codes to the shelter. Asgore and Undyne know part of the code, Toriel may know either the police or church's part, Carol and maybe Rudy know the mayor's part, Alvin and Gerson know the church's part but Gerson, as far as anyone
maybe even the Knight knows, can not speak any more. But if the Knight doesn't want the Roaring to happen, and the heroes don't want it to happen, why is the Knight being so secretive? Why kidnap people at all? The only reason would be that there's someone that knows the prophecy, that is watching the events of the game play out, so they can strike just as the prophecy reaches its climax. That would explain why Kris is helping the Knight. If they're both trying to work to defeat this hidden foe
maybe Gaster, it would make perfect sense for them to work together in a way that makes it look like they're playing out the Prophecy as it was told, where no observer could figure out their plan. This is assuming that (you) are Gaster's listening device.
80s kids fantasy adventure plot that's rapidly unspooling into chaos as it becomes more and more obvious that every single major character is an unreliable narrator
so...yiik if all the characters were a decade younger and if the author wasn't a murakami fanboy