User blog comment:13SIlent/Monster Lord/@comment-24439699-20140214071148/@comment-4991168-20140214113309

It says so in the game. Alipheese the 1st will return once a sufficient amount of monsters will be killed and their energy will be brought back into the flow of the world. Killing a monster = more dark energy returns to the flow. Same applies to angels. The logic is just a little different than what you were applying is all.

Even if it was not, Illias at the end of the game is a construct of light AND darkness so killing her affects both in similar amounts.

So basically :

Many monster alive : Lots of dark energy on earth, little in the flow of the world, no consequences noted if excess light energy in the flow of the world.

Many monsters being killed with not many angels being killed and nobody to absorb/manage the excess dark energy : Lots of dark energy being returned to the flow of the world. Primary noted consequence : Revival of Dark Goddess Alipheese.

Many angels being killed : Little holy energy in heavens/earth, much returned to the flow of the world, consequences unknown.

Current Situation : Illias dying returns much holy AND dark energy to the flow of the world. Most angels and many monsters are alive and their numbers will continue to increase from that point.