It doesn't understand why Sans is angry.
“THE DOCTOR WAS WICKED. HE WAS VERY MEAN.”
Sans ignores him, shuffling more than walking behind it. Despite his silence, it continues taking to him.
“I JUST WANTED TO BE FREE.”
That makes Sans wince. He had helped it gain that freedom. He had killed his father by doing so. He stared very hard at the ground as they slowly make their way through waterfall.
“IT’S BEEN A VERY LONG TIME SINCE I GOT TO SEE THE WORLD.”
“ain’t much to see,” Sans mutters, taking a bit of vicious satisfaction in the fact that it’s still trapped. It’s cage is bigger, but the world is still outside its grasp.
“IT IS A LITTLE SMALLER THAN I EXPECTED,” it comments, tone brightening. “BUT IT IS MUCH LARGER THAN THAT ROOM.”
“this isn't the world! it’s just the underground,” Sans cries, annoyed that it doesn't even understand that it should be disappointed. “we’re all stuck here. you're still trapped!”
Papyrus falls silent, though his steps don't falter. Sans continues as well, too scared to make a break for it.
“WHY ARE WE TRAPPED HERE?” Papyrus suddenly asks. Sans looks up at his back in surprise.
“you don't know about the war?”
“NO?” Papyrus responds, but he doesn't sound all that sure of his answer. “I DON’T REMEMBER MUCH ASIDE FROM THAT ROOM.”
Sans falls into thought this time, imagining an existence that doesn’t extend beyond four walls. Disgusting, unwelcome sympathy wells up again. Papyrus betrayed him. He doesn't deserve such sentiments.
“WHY, SANS?” Papyrus asks again, and there's an edge to his voice.
“because we lost,” Sans says. “the humans won, and we lost. so they locked us in here.”
“HUMANS…” Papyrus draws out the word, like he's feeling it out. “THAT WAS VERY MEAN OF THEM. I DON’T LIKE THE SOUND OF THESE HUMANS.”
Sans bites back a comment about how he and his father had been looking for a way to get past the barrier. Papyrus has set back the research on the topic years.
“IF I MEET A HUMAN, I SHALL MAKE THEM MY PRISONER AND SEE HOW THEY LIKE IT.” Papyrus was rambling as they walked. Sans nods along, lost in a train of thought.
Papyrus said that the strange power he had used on Gaster hadn't killed him. If he wasn't dead, than he was at least gone. The hole he saw must be some kind of gateway. That might explain why Papyrus was such an integral part of the study. Magic like that could help them move through the barrier.
But a gateway to where?