Spoilers for anyone who hasn't completed the game, or played Genocide route.
Brief mentions of suicide, torture.
Sometimes, Sans can convince Papyrus not to go outside on that day.
Papyrus usually ends up sneaking off anyway, curious as ever as to what his brother could be hiding, and the human catches him.
Sometimes, Sans breaks down once he realizes he's gone. Other times, he sighs. He sighs and goes to the place where the battle takes place every time to go and collect the dust covered scarf he knows is out there.
One time, he catches Papyrus as he's sneaking out the door, and breaks down in front of him.
It all just comes tumbling out. The resets. Gaster. The human. Determination. Experiements. He leaves out the part of how many times he's had to go collect his ashes, and how many times Sans has killed himself and their ashes mingled instead.
Papyrus doesn't understand. It's confusing to him, and it hurts Sans even more. Papyrus is simple. Papyrus is sweet. Papyrus doesn't need to have these things weighing on his shoulders. Its good he doesnt understand, or else he probably wouldn't smile anymore. He knows this, but it hurts anyway.
But when he asks, "BROTHER? WHAT IS HURTING YOU SO?" All Sans can reply with is "Humans," and a laugh.
It was obvious Papyrus was expecting a pun or something reassuring if the surprised look on his face meant anything.
Papyrus lives this time. He stays inside with Sans the whole day. Papyrus is determined. He doesn't understand whats going on. He doesn't, and San's knows this, but nothing can sway him. Sans always thought he was the protective one.
When the human comes to them, in the castle hallway, they seem surprised to see Papyrus there. One of the few times they showed any feeling besides murderous glee. He's getting ready to begin his speech, the one that has long grown tasteless and feelingless in his mouth, when his little brother cuts in.
"HUMAN. I BELIEVED IN YOU. I WANT TO STILL BELIEVE IN YOU. ALAS, I CANNOT. THE GREAT PAPYRUS IS CAPABLE OF ACCEPTING MANY THINGS, AND SO ARE MY MANY FRIENDS, BUT..MY BROTHER IS NOT VERY GOOD AT THAT. MY BROTHER IS LAZY. YOU HAVE MADE MY BROTHER VERY UN-LAZY..I..I DO NOT LIKE THAT."
Papyrus wasn't ever really good with words, but Sans feels every bit of feeling in them.
"YOU END HERE, HUMAN. I AM SORRY WE CANNOT BE FRIENDS. WE CANNOT BE FRIENDS IF YOU ARE NOT FRIENDS WITH SANS, IF YOU HURT SANS."
The battle begins, and Papyrus dies oh so quickly. Sans is glad for that. Sometimes, when he'd go to collect his brothers scarf long after the battle is usually over, he finds the human simply cutting tiny bits of his brothers bones. He had to put his brother out of his misery once.
Sans is glad for this quick death. He'll be joining him again very soon.
Still, not quick enough for there to be no last words. They come from his dying little brothers still smiling face, and it hurts. He begs for Sans to tell him everything ( apparently he'd figured out life repeats ). Tell him everything again, and again. He doesn't want his "COOL BIG BROTHER TO EVER BE ALONE!"
...
When Sans wakes up with the new reset, staring at the ceiling of their house in Snowdin, he decides to tell Papyrus nothing. Tell him nothing ever again.
Papyrus not believing in someone, not wanting to be friends with someone, not being happy.
Papyrus having to end someone. No.
That Sans' own personal bad time.