In terms of game design, I think sans bleeds unlike the rest of the monsters because they just turn to dust, leading no real damage to be seen, maybe a slash or 2, undyne melting a bit (but after the true lab we kinda got used to it). Sans showing a physical amount of damage in a realistic way kinda gives the player that "OHH Shi that was too much" feeling that genocide is known for. Just one last kick in the teeth from sans.
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