King resented the lightners for abandoning his people, the toys in the unused classroom, and intended to destroy the world in retaliation. Queen still intended to help lightners, but because she's the internet, she couldn't understand their preference for freedom and tendency toward self-torment, and didn't really grasp the consequences of this imbalance between light and dark.
The Roaring Knight is the one opening up fountains, but it seems that merely opening them is the end of their involvement, whether or not any darkner understands the significance of the act. Queen was able to infer that any lightner could in principle open up a fountain, because lightners possess Determination (there's that word again). Berdly was about to try when Ralsei stopped him.
The ending of chapter 2 seems to hint that Kris, when not under the player's influence, is the Knight, but at the same time introduces another possibility, that there is no single Knight. Then again, Kris is the only human in town, and while the significance of the correspondence between a human soul and player control as indicated by a little red heart is something the game doesn't want us to forget about (especially with all the ominous Undertale references all around), it also hasn't really given much of an indication of its meaning in this context.
Appearances can be deceiving. The game wants us to worry about Kris, about what kind of troubles they have and about what they get up to when we're not driving them. But the same twisting of expectations that makes the jokes land so well also means we can continue to be surprised at how anything dramatic plays out.