I HAVE A DELTARUNE THEORY.
Okay, so take the fact that it's canon Kris is being controlled by the player. Kris is a separate person, who only occasionally has control over their actions. They woke up one morning being controlled, and have been unable to escape from this nightmare. What if when they tear their SOUL out, they're trying to stop you from controlling them? Everyone knows it's impossible to live without a SOUL, which rules out the possibility that the SOUL belongs to Kris. So what if it's a Chara situation, but with a living player instead of a dead character? Someone that constantly watches them, keeping watch over their shoulder. They cannot separate from Kris for long, because they are tethered together. Much like Chara was tethered to Frisk. And when Chara caused events by themselves (Killing Sans, taking one more step, etc.), that's what we do. Frisk had no control over that, and neither does Kris. We are the Chara of this world. So in these events like the SOUL-tear, we have no control. Kris is acting of their own accord. They're trying to stop you. They hide you and run away. They try to use your SOUL to fuel the new fountain, and presumably destroy you. Freedom. Freedom is something many characters desire. Jevil, Spamton, Kris, basically all Darkners. But let's roll back to Spamton. He is much like Kris, and it's obvious they see themself in the other. After the battle with Spamton NEO, when you cut Spamton's final wire, he falls. Dead. Unable to survive without some sort of strings attached. These strings are a common way of representing the loss of control. Being a puppet. It's confirmed that Kris was jarred by Spamton in general, because of the puppeteering that he reminded them of. He reminded them that nobody truly has control over themselves. Nobody can choose who they are in this world. Everything is predetermined, even if you fight hard to do the opposite. So, when Kris sees what happens, they get home and try to rip out of your control once more. They pull you out and try to escape, slashing Toriel's tires as an excuse for anything bad happening because of the player. And they get back with a plan. After preparing, they let you take back control. But then, in the dark of night, they tear you out again, and sacrifice your SOUL to the dark fountain.
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