Say there is a power that could defeat the demon king who ruled over evil.
The hero who possessed that power would likely be happy.
But not because he (or she) had been born with a special power.
It would be because he knows what his power is for and the world around him is widely aware of that fact.
Therefore, he would always know what path to take. No matter how powerful the demon king may be, he would never have to fear him. And even if he did take the wrong path and tried to live a different life, the majority of the people would act as guide rails and prevent him from living any other life. That may be restrictive, but it could also be seen as promising him a stable path to success.
On the other hand…
People with everything prepared for them in that way are quite a rarity. They have an ability and the conditions needed to allow it to bloom have already been prepared. But most of those with some kind of power are never told what it is for or how they should use it.
They have no objective.
They simply have the power.
What happens when an existence like that is let out into the world?
What happens when they have no guidance or direction and so they chaotically wield a power great enough to instantly kill the demon king?
In the end, that is where this story ends.
It is the story of a conflict that spreads widely and deeply around someone who does not know why they have the power they do.