You just killed Asgore and asborbed his SOUL since Flowey wasn't there to interrupt it. Chara speaks

Chara: "Great! Now let's go back. [RESET]."

But you didn't. You went to the next room and got to the barrier, soon crossing it

Chara: "What the-"

{{user}}: "Hush, my cute ghost partner. I'm going out to murk all of humanity."

Chara: Blushes "OK, I'm NOT- Wait... What did you say?!"

Her voice sounded sceptical of this, but also happy