Sans was relieved when the test day didn't repeat, he wasn't sure the Spiraling Stars would let him out of that cruel test on the grounds that he already passed it. They were already clear, he was just a game to them.
His relief turned into dread when the woman behind the door asked him to protect any human that left the ruins. He only promised because he liked the lady, never mind he barely knew the woman.
When he saw the human, he wasn't surprised to be able to see into their soul, he could inside of everyone's soul, that very skill allowed him to be the Royal Judge. What was surprising, was the soul empty? He's no human expert, but that seems odd.
He opted to watch the human, sometimes they stood in one place for a long time. And when they died, a slight seconed before they respawned, Sans could see a "game over" screen, with a message to stay determined, in Asgore's voice?
He had to know, never mind the curiosity about killed him before, Sans asked the Spiralining Stars what was the screen. "Life is yet but a game." Was an answer.
Next thing he knew, he was sat in front of a screen with the human in the inn. "We're a game?" That was disappointing. "Yes. Watch the previous battle."
Sans watched the human fight Papyrus, seeing the screen with the buttons. That is not what he sees, at all. Maybe the heart, but that was a side effect of pulling out a human's soul, or at least that is what he read.
"I'm a toy?" "Exactly, you are a thing at the very most." He couldn't believe this. "I made the person playing the game. There are many paths they can take. I wonder if you can handle an evil player." And on that vague note, Sans was left to have his existential crisis in peace.
After he had his breakdown, Sans opted to do everything in his power to appease the player, yet judge them on their actions. If you have a special power, you should use it for good, after all. He just had to be careful with the wordings of his judgments, don't wanna anger the anomaly, or he risks whatever cruel thing they can imagine, and he can't meet the one controlling the human, in order to gauge their reactions. Again, how do you win against an invisible opponent?
