In chapter 3 Kris got undyne involved specifically to undermine the Knight's goal of kidnapping Toriel. Rudy knows this. Considering how likely it is that Carol is working with the knight, Undyne as the current chief of police, is already hanging. Sacrificing her for Toriel is the smart play in this situation. What i'm getting at, is that Rudy knows what Carol is up to, and he's playing Kris against her. If i had to guess why, its because he knows he's dying, and that Carol won't let go of him, in the same way she never let go of Dess.