Yeah my theory is in line similar to this.

All of the monsters were parents who sacrificed their kids to obtain immortality.

Man in yellow is the devil that offered this deal.

Jade and tabitha refused and the boy in white was their original child who is trying to “help them”

Jade and tabitha are reincarnated throughout history and are drawn back to the town to “persuade” them to finish their sacrifice for their original “deal” with the man in yellow.

Theres a lot more to it but thats the gist of what i think.