First of all, it's implied that none of them had enough Determination to LOAD or SAVE, meaning they only had one life to live.
Gaster and his followers are characters hidden in the game's code with an unclear identity.
Turns out the child from the opening movie isn't the character you're playing as, and in a Genocide run, that first Fallen Child destroys the world.
Problem is, that boss is built around trying to make you feel hopeless, so if you know how to beat him going in, the effect is ruined.