"PS. And a dualist god is irrelevant to us from a rational standpoint because there is no way to know whether the good or evil dimension is 'speaking' to us - an evil god lies, even if evil only in part. From a rational standpoint, a god who is altogether evil is no different from a god who is altogether not - then it's just semantics - and he is irrelevant for the same reason."
Our current concept of "Evil" comes from the idea that God tests us. (Like a pop-quiz for your soul) He employes Satan (against Man, not against God) to make a case against us.
In other words, "Evil" is simply "not Good" and so God cannot be Evil, as to be Evil is to be Not Godly.
Evil is the lack of Good, just as darkness is the lack of light.
There could exist nothing but that which an omnipotent, omniscient god considers good, because the absence of anything would also be a decision of said god. In other words, if he considered the absence of something to be evil, it would no longer be absent unless he were evil at least in part, because to permit its continued absence would be to engage in evil.