He was a failure––the single thought found its way into his mind as he limped down the hall. The thought was normally a pernicious one to most, but, to him, he was used to it. He had been acclimatized to the thought that, no matter what he did, someone would be disappointed. So, no, he couldn't say he was upset, even though he felt he should have been; rather he felt a calm resignation, a lifting from his ribcage. For the time being, he was free.
He finally had to stop his limping walk, his legs too shaky to continue. He landed on his patellae, managing to keep himself up for about another second or two, before he finally completely landed onto the floor, his zygomatic bone on the cool tiles. He could feel his bones turning into dust, and he could feel his consciousness slipping. The already lit corridor was becoming brighter and brighter, whiter and whiter.
A low, mirthless chuckle rumbled through his dissolving rib cage, causing some of the dust to flutter and flit into the air. Had someone asked him long ago how he thought he was going to die, he never would have guessed this; he wouldn't have guessed any of this, honestly. Now, however, it was a part of his life––for now and for eternity.
"See ya, Papyrus," he whispered out before the entirety of his being turned into dust, ending the life of Sans the Skeleton.
(For now.)