(I will be referring to Kris as a male in this theory. Also, read the whole thing for it to make sense.)
So Kris has been living in Asriel's shadow the entire first chapter. Everyone's talking about Azzy, but not Kris. Also, you can very, very slightly move the soul in the final scene of Chapter 1. Kris wasn't doing the adventure. You, the player, were doing it. My guess is he wanted to kill Asriel (hence the knife incident), but you were in control. So he ripped you out of his body.
Toby Fox tweeted some teasers for Deltarune on his Twitter account. One of them was a scene with Kris in the city. Based on what Toriel said in the car in the beginning, the university must be in the city. Kris might want to go to the city to kill his brother everyone talks about. But in the image Toby tweeted, Kris is wearing his Dark World outfit. Is the city in the Dark World? Maybe. But Asriel probably went to university there... And in the Light World, everyone's talking about his disappearance. He might have recently appeared in the Dark World. Seam talks about a strange Knight who recently appeared in the darkness.
Could Asriel be the Knight?
Wait, now that I think about it, Kris might actually be doing a good thing killing Asriel. But then everyone would be sad. I don't know the future, we'll just have to wait for the rest of game...
I am sorry, but I just wanna share my theory: Bidoof is the knight. And I have a very logical reason for this. First, this image proves everything: https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&ved=2ahUKEwjru_jP-ermAhUXLs0KHVzqDikQjRx6BAgBEAQ&url=https%3A%2F%2Fwww.roblox.com%2Flibrary%2F25750541%2FBidofo&psig=AOvVaw1kjVTX6x4-dWFjwvujj9xI&ust=1578261765671907
Second, B I D O O F I S G O D.