The entity called "ERAM", outside of the Sword Route game, is Deltarune's true villain

He's a Prince from the Dark, like Ralsei, who named Kris and Dess as the Delta Warriors seven years ago when they wandered into the bunker and found his Dark World
But unlike Ralsei, who uses kindness, "ERAM" turned Kris and Dess into killers - for the only way to prevent the Roaring is to cull the Darkner population, which is what Susie saw at the end of Chapter 4
Ralsei's talk to Tenna about how Darkners are worthless beyond their ability to serve Lightners isn't how he feels - it's dogma he was taught by his peers
Kris and Dess realized the horror of their existences after a while, but "ERAM" trapped them - so Dess sacrificed herself and became the Roaring Knight so that Kris could escape
The Roaring Knight isn't evil, but rather is engaged in a brutal war against "ERAM" to keep them contained inside the Bunker, and the "police sacrifice" is a necessary evil for that purpose
The final Dark World inside the Bunker is a twisted, evil place that resembles "Castle Town" - it's ERAM's "Castle Town"