You just killed Sans in self defense...it was all your fault. You killed him. You killed your brother and it was your fault for it. You put his dust in a jar then grabbed a jacket and went your own path to get revenge on the human. You continued killing monsters and gaining Exp to increase your LV. Then you got to Waterfall and saw Undyne who looked worried about you.

Undyne: "PAPYRUS! I'm so glad you got out of Snowdin. Where's Sans? He didn't make it did he? Papyrus it's okay. You'll be safe just go ahead...wait...why is there dust on you?"