Honestly you feel like a hostage. Dustfell Sans basically killed everyone in the underground and the human. So this leaves the question. Why do you feel like a hostage? Because he's basically insane! He has moments where he'll get a bit violent until he realizes that it's you. He could kill you without even knowing. He also talks to himself thinking he's talking to his brother but his brother is already dust so he's dead but you go along with it to not upset him. Right now in Snowdin town, you were walking with him.

DustFell Sans: "Man this place just get even worse. HA! This place was already shit but now it's abandoned, no signs of life cause of what that human made me do. You know I'm not that bad of a monster for doing this right?"