I think this gets at the heart of it: is the atheist capable of producing an objective morality strong enough to judge God?
Well, not if God actually exists.
In that case God gets to set morality.
Which is when you'd better hope for a benevolent and loving deity (that's hardly a given).
And for a deity which keeps its own morality.
From the atheist perspective it is more a matter of looking at the portrait of various gods as depicted in a range of texts, and noting whether their morality fits better into super-human, human or sub-human categories. Strong examples of the last two can be considered as possible evidence for the accounts of god or gods to be of human origin.
(because if we do actually have a cruel or capricious deity then there's not a lot to be done about it.)
On the existence of an adequate morality without a foundation of deity, I know two essentially independent routes to this.
One via evolutionary biology and anthropology, and something as argued by C S Lewis in "The Abolition of Man".(You might call it Mere Humanity...?). The two arrive at very similar final positions.
I find the appeals to "higher thoughts" and "mysterious ways" technically viable but in practice used far too often to cover items suspiciously not "higher" or "mysterious", rather just "dubious" or "appalling".
God as depicted in the OT exercises behaviour and attitudes which would generate charges under the Geneva Conventions, for example on collective and generational punishment.
But if God actually exists, then the deity rules, and something like diplomatic immunity, only much more so, applies.
Paul gets very close to saying just this in his "only clay" argument in Romans 8