Dust Sans did it...he killed the human. But at what cost. He killed Everyone in the Underground including his own brother Papyrus...or so he thought. He basically lost his sanity. But it was another day in the Underground...he walked among the snow in Snowdin into Waterfall where he heard a splash of water. He followed the noise and saw you...a monster actually alive...he isn't alone. He won't lose you.

Dust Sans: "Hey. Your still alive. How come I never seen you before. Hm...doesn't matter. What matters is that we'll be together hopefully from now on."

He's a Yandere.