The fact of demons refutes that God can be both "Omni" and Good

What seems to me a very powerful argument against the mainstream orthodox Christian notion that God-the-creator is both Good and an Omni-God; is the theologically-assumed reality of demons - that is, of never-incarnated spirits who are irredeemably committed to evil, and who operate in this world.

In an ultimate sense, this is a just a subset of "the problem of evil" - and its existence At All; but most of the attempted explanations of the presence of evil in this world are focused upon human beings. Apparently; many mainstream Christians (although not me) are sufficiently satisfied by the traditional explanation that human evil is a consequence of 1. human free will (granted by God) and 2. The Fall; plus 3. that there is always a possibility that mortal humans may repent and accept salvation. 

It is this possibility that any human who is currently committed to evil may, nonetheless, repent and accept salvation that serves to explain why such humans are not annihilated or incarcerated somewhere where they can do no harm. 

So; most orthodox Christians believe that evil demons are real, and that they have a harmful effect on this world; but that they cannot ever repent. 

This creates a contradiction. 

It seems obvious (?) that any Good God who had the power to do so (as an Omni-god does, be definition); would decisively sequester such evil beings somewhere where they could not corrupt or damn human beings. 

That God does not do; and allows demons access to humans, implies either that God is not Good, or that God is not "Omni". 

[Note added: To clarify, here I am not trying to refute the orthodox explanation of the existence of demons in the first place. This, in traditional orthodoxy is usually a matter of angels being conceived as a separate creation from Men. Upon having been created, these angels then use their free will to choose evil - they become fallen-angels. This fall is then regarded a - according to the attributed nature of angelic beings - a permanent condition. What I am instead saying here is focused on the question of what God does about fallen-angels/ demons, after they have arisen.] 

Maybe this is why so many modern Christians who cleave to the definition of God as wholly Good and "Omni" tend to deny the reality of traditionally-understood demons. Instead they (seem to) regard demons ("if they really exist") as merely confused, misguided and "disturbed" spirits who might attain salvation; or in some other way as only-partly-evil, or reversibly-evil.   

Or else such Christians fall back on the "God's Goodness is total but un-understandable by Men" argument; which leaves the Christian "God-is-Good" deity indistinguishable from the Islamic concept of "Good is God".  

Such are the ways in which the stark contradiction of traditional Christian theology when it comes to demons is obscured; and concealed behind imprecision, and complexity of (only vaguely-understood) abstractions. 

NOTE: I intend to present an alternative metaphysics to explain permanently-evil demon-spirits and their continued operations in this world; in a soon-to-be-published post - which will be referenced back to the above argument.