I've often wondered why it is that, once Descartes believes he has proven the existence of God, he never mentions his previous hypothesis of the evil genius, spirit or demon. He brings up the dream hypothesis in the Sixth Meditation and mentions of sensory and memory errors are sprinkled throughout the Fourth, Fifth and Sixth Meditations. So, he must think that the proof for the existence of God constituted a disproof of the existence of the Evil Genius. But why should that be so?
One of those paradoxes involving the existence of God is the question: Can God create a stone so heavy that He cannot lift it? It is impossible to give an answer to this question without questioning an assumption of the question itself. If God cannot create such a stone, then that failure is a limit on his power. If God can create such a stone, then His failure to lift the stone is a limit on his power. So, either way, God's omnipotence appears to be impossible.
The response to the puzzle is that for someone to be omnipotent is not for that person to be able to do anything but to be able to do anything that is logically or metaphysically possible. Even God could not make it that I do not exist at the moment that I am thinking. But this is not a limit on God's power since it is metaphysically impossible for me to think and not exist.
So, here's the puzzle. Suppose we think that God exists, why is this immediate proof that the Evil Genius (EG) does not? Presumably the reason is that the existence of both God and EG is metaphysically or logically impossible. But why are these two things logically/metaphysically incompatible?
Could it be that God, being omnibenevolent, would want to destroy all evil, and, the EG being evil, God destroys him (or never lets him exist in the first place)? That might work except that God allows some evil, so we cannot say why God would allow some evil but not the EG.
Moreover, if the EG has all God's power and omniscience but not His goodness, then perhaps God does not have the power to destroy him. The Manicheans thought that there were equally powerful good and evil beings struggling for control of the universe. Perhaps God and the EG are, similarly, equally powerful and knowledgeable beings.
Perhaps the argument is that there can be only one creator of the universe who holds everything else in existence through his concurrence--his decision to keep it all in existence. But why think that the EG depends for his existence on God? It's at least plausible that the EG is an independent, necessary existent just as God is supposed to be, and God could not logically be required to maintain him in existence. Why is this? Well, run the ontological argument not for an all-good being but for a most evil being. An evil being that exists is clearly much more evil than a being that does not exist. Hence there is a most evil being, the EG. As to the idea that God created everything. That need not be the case. Perhaps God created everything good, and the EG created everything evil as the Manicheans apparently believed. That's at least as plausible a response to the problem of evil as anything else I've heard.
In short, Descartes made no attempt to prove that the EG does not exist, yet it is not at all obvious that the existence of God logically entails the non-existence of the EG. One cannot rely on God's supposed omnipotence to rule out the existence of the EG since that omnipotence does not cover cases of logical or metaphysical impossibility. And it is at least as plausible that the EG is a necessary existent as it is that God is a necessary existent. So, there's clearly a gap in Descartes' reasoning, and I don't know how, given Descartes' resources, to fill that gap.