-Lufas got too strong
-Demon Lord realized Lufas would win and survive (no self-sacrifice)
-Demon Lord told Lufas about the Goddess' Plan
-Lufas worked to break the Goddess' control on the world
-Goddess tempted and controlled all the Heroes, except Loli Vampire, to seal Lufas
(We actually SEE the Goddess tempt Loli Vampire in Chapter 34, Page 38. Once you accept the Goddess' Temptation, she can fully control you)
-Without Lufas and Loli Vampire, heroes got asses beat
As to WHY the Demon Lord told Lufas
He doesn't want the Demons to exist solely as the Goddess' spell. He managed to sire a half-demon son who exists outside the Goddess' control. So even if the Goddess nixes the spell and erases all demons, his son Terra will survive and the Demons won't go extinct.
SO basically The Godess lost control over her playercharacter in her videogame that she created herself. hm.
and she is a cheater so she is kinda cheating. so she is basically a gamer.