God is not the Creator of evil because He is good and He is holy. But God did create Lucifer who was perfect in his ways until iniquity was found in him (Ezekiel 28: 15). It is from this we learn that evil came into being via the personhood of a created angel. Because evil is a transcendent entity, having its origins in the spiritual realm, its existence therefore proves the existence of the spiritual realm. Because it was not created by God, evil should not therefore exist, so then it exists in defiance of and in opposition to the created order of God; in other words, it exists in defiance of existence.
The personhood of evil is better understood when we realize that it is not simply a force like that of an earthquake, a hurricane, or a fire. All of these bring calamity and destruction, but none of them constitute the true essence of evil. These are impersonal forces that act without consciousness or of their own will, whereas someone such as a serial killer commits his acts with conscious intent and of his own will, and on that basis is therefore evil. Thus evil is not an impersonal force, but a personal one that has consciousness, will, and intent, and is why we are led to the truth of personhood in identifying the source of what evil is.
Evil is just that because it is an inversion of all that God created and instituted; it is by nature inverted, and its nature is to invert. It will twist and blend and turn truth, reality, and existence in upon itself. It is an anti-existence, an anti-being, and its nature is anti-nature. It has no true or absolute existence, and therefore it acts to bring that which exists into non-existence. So evil by nature is a destroyer because it opposes everything that is.
Evil is parasitic because it possesses no absolute or true existence of its own. It is not of the created order and can only exist by drawing its existence from that order. Evil must use the good, and so although good can exist without evil, evil cannot exist without good. Truth can exist without falsehood, but falsehood cannot exist without truth. Laws can exist without crimes, marriage can exist without adultery, and life can exist without murder. But crimes cannot exist without laws, adultery cannot exist without marriage, and murder cannot exist without life. Destruction requires structure, immorality requires morality, and sin requires that which is holy. The good is always primary, but evil is the parasitic inversion of the good, and as an unwilling witness, it inadvertently testifies for the existence of the good and to the existence of God.
God did not create evil, but He did create personhood, consciousness, and free will. Then when we speak of good, we call it that because it is chosen of our free will; and if good is a choice that must be made, then its opposite (evil) must also be available for the choosing. Having already established that evil is a conscious entity in the personhood of Lucifer, we therefore learn that he was a created being who by his own volition turned against the created order; he turned against existence itself and became an anti-being.
Humans and angels are the only two entities with the ability to choose between good and evil; and because evil is spiritual and beyond flesh and blood, its origin is found in the realm beyond flesh and blood; its origin is in the spiritual realm of angelic beings. It is there in the realm of created angelic beings that we find one angel who turned against the fabric of the created order and against existence itself. Lucifer was that conscious being with free will who pulled down a third of the angels (Revelation 12: 4) by rebelling against all that God had created and instituted, and whose influence is a parasite that attempts to hijack all that is good and bring it into non-existence.