My specialty is not philosophy of religion but rather epistemology. No doubt the following argument has already been made by someone or other in the history of philosophy, but it may be useful to someone else precisely because it refrains from some of the more (to my mind) esoteric concepts in the philosophy of religion. This argument will make no use of phrases like "no distinction between essence and existence" or "metaphysically simple." Whether the concepts are there under some other guise I leave for the reader to judge, but the idea is that the argument will be accessible to those, including myself, who don't find some of those Thomistic notions helpful. It will become fairly clear that this argument owes a bit more to Platonism than to Aristotelianism.
So here goes:
To show, by reductio ad absurdam, that there cannot be an evil being who is the self-existent First Cause of all else.
Suppose that there were an evil being who was the self-existent First Cause of everything other than himself.
Then, there must be such a thing as the Good, independent of this evil being, against which this evil being sets himself, which he hates and rejects, for otherwise it would be meaningless to say that this being is evil.
The Good, therefore, is conceptually and metaphysically prior to the evil being.
The Good cannot be of the essence of the evil being's very nature, for if that were the case, it would be meaningless to speak of his rejecting and rebelling against it.
Therefore, since the Good which he hates is greater than he is and exists independently of him, the evil being is not actually the self-existent First Cause of everything else.
We have derived a contradiction from the supposition that there could be an evil being who is the self-existent First Cause of all else. Therefore, we conclude that there cannot be an evil being who is the self-existent First Cause of all else.
The resemblance to the Euthyphro dilemma is fairly clear. One horn of the Euthyphro dilemma is that, if God (or "the gods" in Plato) loves the Good because it is the Good independent of himself, then the Good is greater than God. (The other horn is that goodness is arbitrary if we merely call it "the good" because it happens to be what the gods love.) The traditional Christian response to the challenge is to postulate a tertium quid--namely, that Goodness is real and is of the very essence of God's nature, not outside of Himself. Therefore, it is impossible that God should love that which is evil, but not because God is holding Himself to some standard outside of Himself. That response, as noted in the argument, is not available if one is postulating an evil First Cause, since the will of the evil First Cause is contrary to the Good.
It may be of some minor interest to readers to know that I got thinking about this by using the analogy of a human being who maliciously wills to make computer viruses. I proposed that analogy in a Facebook discussion and got some useful responses. Obviously, the will of the malicious hacker is an evil will, and he may be a real genius at creating computer viruses. This seems like a counterexample to a principle such as, "Evil cannot create," which might be used to counter the possibility of an evil First Cause. It then occurred to me, however, that the concept of a computer that runs correctly is necessary for the creation of a virus that harms the computer. In fact, a computer virus couldn't be instantiated in the real world at all and fulfill its unpleasant telos if there weren't some computer for it to attack. Therefore, the proper function of computers is both conceptually and metaphysically prior to computer viruses. Hence, the will of the creator of a computer virus, as a will to do evil, is derivative--it is a will to harm and subvert something good whose goodness is a reality independent of his desire to harm it. His "creativity" itself, therefore, takes a form that is derivative, parasitic upon the goodness he wishes to undermine or destroy.
That thought then inspired the analogy to the Euthyphro dilemma.
See what you think, gents.