So. Frisk feels pain with every death (presumably). After death, he always appears in the void and "rests" there. At death 270, he had a dream about a school massacre, which he most likely caused himself. But in reality... essence? It's hard to say.
Oh, and he also loves his friends, which is nice. Frisk doesn't like anyone in the underground because they are all stupid and think he is weird. He dies for them every time... But why? What's stopping him from simply not helping them? And for some reason, death 272 is skipped.
As far as I can tell, monster food is made of magic and is not perceived by normal people as proper food. It heals wounds, but does not satisfy, perhaps, for a human it does not even have a taste.
oh... He... god. where did he get a gun? Or is it a yellow soul gun? Hm, why he even have a gun???
Okay, someone's blood was spilled there. The handwriting is not very legible, but I assume Jake was with his dog. It's possible, it's possible: the dog bit one of them or was injured. Jake pushed Frisk. On purpose? By accident? Tom was watching. Wanted to help? Stop? Hmm...
A possible scenario: Frisk and Tom were having fun in that cave, as usual (as we remember from the comic, where entry is prohibited). Then Jake shows up with his dog (?) and a scuffle ensues, resulting in Frisk being pushed into the underground.
The question remains: was it on purpose, planned, or an accident? Regarding Frisk's dream where he finds a mountain of corpses at school... Is it possible that this is his secret desire? Presumably, Frisk is an outcast at school, which is why he dislikes the people there.
So far, information is patchy, as are Frisk's memories. Perhaps the underground is an allegory for purgatory? Who knows.