gaster sacrifices himself to save sans from death
The pillar thingie in the ceiling could've hit sans in the head, and crushed his skull
sans was only 12, and was way too young to die
so gaster pushed him down, but the pillar hits him hard in the spine, and knocks him into the CORE
he sacrificed his ability to exist in order to save his 12 year old child
papyrus was at home, being babysat by grillby (who was 25 at the time), so he didn't witness his father's fall
Draw your original anime with iOS/Android App!