On a more story-related note, I've noticed that many people think Sans' decision to snipe you if you spare him was based on anger or revenge, but I think it's more a matter of practicality. Here's my reasoning:
1. If you fall short of the requirements of the genocide route by so much as a single monster, Sans won't fight you, even if you killed as many of his friends and family as possible and the monster you spared was a stranger to him. This indicates that his decision to attack you is based on the need to save the world, not revenge.
2. By this time, he's figured out that Frisk's backward time jumps are triggered by their death. If Frisk wants to spare him, repent, and undo the damage they've done, they'll need to die in order to do it. He can't just let them walk past him, because he still doesn't know how they're going to destroy the world or if his survival will preempt the disaster.
3. Sans' attacks are the most efficient in the Underground, with the possible exception of Flowey and the obvious exception of Frisk. If Frisk has to die, it might as well be at the hands of a person who can get it over with quickly. In that regard, his attack is actually kind of merciful.
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