Heres an idea.

The pattern is not intrinsicaly good or evil. This means that the total amount of evil in the world is always a constant. That no matter what you do the good done will balance the evil. The only way to improve the world is to remove the Dark Ones influence by properly sealing him up. so that it is like in the age of legends where the world was mostly good util the dark on was freed.

By cleansng saidin of the taint, he created a lot of good. But the pattern needs to balance. The death of the amayar provides the balance