Okay, this is a sick au. Kris as Asgore and Berdly as Toriel????
I can totally see Kris being put in Asgore's role, as in Deltarune Kris is working for the knight, but is heavily implied to not enjoy it. They believe they are evil and are hating themselves for not being on Susie's side. I can fully believe that in this au, they got wrapped up in the promise of freeing monster kind that they feel like they can't back out of this war or they'll be disappointing everyone and possibly let their children's death be "meaningless" plus if Berdly takes the role of Toriel in this, it could make them keep going because they already lost Berdly so might as well finish it. (sunk-cost fallacy). Kind of makes me realize Asgore and Kris might be more similar than one make think. (chapter 5 is going to be awesome)
Heck, with Kris being a human, makes me think whoever Frisk is in this au might be the 7th human instead of the 8th (we are counting Chara), and it's revealed that Kris is planning to sacrifice their own soul to the cause, because with everything that they have done, they don't believe they deserve to live anymore and the reason why they haven't done it yet, is because monsters still need them.
As for Berdly, hm... he (or she, I know some people believe she is an egg and I'm not sure if she transition in this au) and Toriel don't parallel each other. Berdly is more connected to Papyrus and possibly even Undyne than her, but: I can see Berdly leaving Kris and trying to stop their plan by preventing humans from leaving the ruins, because in the weird route, Berdly stops the superiority thing the moment Berdly believes Noelle is in actual danger, and starts attacking Kris trying to separate the two. It fails, unfortunately, but it does show Berdly can put aside Berdly's own ego to do the right thing.
I'm not sure Berdly would run away instead of any other method, but maybe Berdly did try at first. Maybe they had an explosive battle, and when Berdly realized Kris was too stubborn to take back what they said, Berdly leaves in the hopes Berdly can prevent Kris from killing humans by protecting any future humans that enter in the ruins. It's even possible by the time, the Frisk stand in comes in, Berdly has regressed in Berdly's character, as all the failures have been hitting Berdly hard along with the death of Berdly's children, that Berdly starts acting superior, because that's the only way Berdly knows how to cope. Does that mean Noelle could be Sans? (Yes spreading my noelle & berdly friendship propaganda) Where she's the only one keeping Berdly out of the deep end, because she's the only person Berdly has left?
Anyways, banging art my tumblrina.