So Vishnu awakened Prahlada, blessing him with eternal life and jivanmukti. Deva-asura fight was resumed and the old order of things was restored so that the universe continues in its eternal nature.
How could God Himself wake up the asura element and bring about constant warfare? Is not Pure Goodness the nature of God?
Goodness is only relative. Good always implies bad also; they always co-exist. The one is the obverse of the other.
Source: Talk 326