It is a classic, and in my view powerful, critique of mainstream Christian theology to find it to hard-to-understand where evil come-from if God is omnipotent, created everything-from-nothing and is wholly Good.
The - apparently unavoidable - implication is that since God created everything, then God created evil. But then God can't be wholly Good...
The best answer is that God wants Men to have free will/ agency; and evil is the price to pay.
However, this answer still leaves open the question of where evil ultimately comes from - how come there is any evil at all in a reality wholly made by a wholly Good God?
Where does that evil even come from? (Where, if not from God?... but then why did God create it?)
On the other hand; I regard creation as a work-in-progress; and each man and woman as having an eternal origin separate from God's creation.
So it is no mystery what evil is, nor where where evil comes-from: evil is that which is opposed to God's creation; and it comes from an agency of free will that is uncreated by God.
God does not need to invent and implement evil; it is something that just happens when Beings choose to act against creation.
The spiritual war is between those who align and (ultimately) ally with God and creation; and those who oppose God and creation.
The alternative to dwelling in God's creation is unorganised chaos - but chaos is not evil, since chaos has no purpose. From the Creator's perspective, chaos is the 'raw material' of potential creation.
But from the perspective of those who align with God, those Beings who purpose to reduce creation to chaos are evil.
And that is the origin and nature of evil.