[UNDERTALE SPOILERS] He never hurt anyone
There's an enemy called Shyren that you can hum with to start a concert. If Papyrus is alive, then Sans will show up to sell tickets to the concert. If Papyrus is dead, instead a hooded figure will be watching you. It's possible the figure isn't Sans but I think it's really unlikely.
Which means that if you spare Shyren at that point, Sans will be watching. Can you imagine how that would make him feel?
broken font hearts on the pavement
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