Also, I think there is a theory (at least I saw the title of a YouTube video but didn't watch it) that the human dies in the Neutral Endings.

Considering Sans calls the human and says "is anyone there? guess i'll just leave a message," I actually find that theory possible.

After all, the human can't leave with just a human SOUL. They also need a monster SOUL, which they did not get, as Asgore's SOUL was destroyed by Flowey and Flowey either runs away or gets killed, either way that SOUL is not taken either.

Which means the human is stuck in the Underground. Maybe they tried to leave through the Barrier but failed and maybe the battle against Asgore/Flowey became too much for them so they fainted/died? Okay, now I think it might make more sense if they fainted into comatose instead of actually dying.

ANYWAYS, that makes any Neutral Ending sadder, along with the standard "An Ending" theme, especially if Papyrus is alive and just talks to you, even though you're not there anymore, just like he sends voice mails to Undyne without knowing she's dead. ;_;

This game has so many sad sides.