It is odd to see this attributed to an individual. It actually goes way back to religious Neo-Platonism and Gnostic Christianity variants like Bogomilism.
They had the experience from extended harsh asceticism that acceptance is the way of the weak-minded and renunciation is the best, perhaps the only, way to have free will. Therefore, Satan must be in control of the world, since humans become their best only when they renounce it. So, the creator of our world, the entity that we think of as God, is really and basically evil.
Not only did they notice this problem, they acted on it -- any Church who considered the Creator worthy of worship needed to be abandoned, and they left and prayed in the fields, they wrote secretly and instructed their disciples individually.
Their later Neo-Platonic apologists proposed what I see as a very creative solution.
In the light of independence as the root of the true Will of God, Satan's renunciation of God's authority was the assertion of his own free will, which he had as an aspect of his being the very best and most complete of the Angels. The others could remain good only because they lacked something essential to real good.
In that framing, not only does the Satan, our evil God, exist, but his creator must also, against whom this world is an act of rebellion. If there were not some disagreement between Gods, or within God, good and evil would have no clear meaning.
They deduce from this apparent contraction that there are layers of worlds where basic evil in one is a central aspect of good in the next one up.
(And in their writings on this we have the Apocrypha of modern Satanism leading up to Crowley, Thelema, Sethianism and all that. The original, "unelaborated" Apocrypha were rediscovered at Nag Hamaddi, and said something quite different. But it is traces of this more creative form we find in the Zohar and the Tarot of John Dee.)
I think of this as the ultimate seed of Hegelian dialectic. We attain a better, more complete notion of good by relativizing what our world sees as good and what it sees as evil in the right way. That always means that what was once evil is, in a better way, really good.