I posted a much longer reply to Craig's argument here, and I'm writing a long but shorter new reply in the form of a hypothetical debate between a defender of the metaethical argument, and a non-theist.
But while I'm working on that reply, I thought I'd post a few, brief objections in much shorter posts. In this post, the objection is based on an argument that mirrors Craig's metaethical argument: the metacruelty argument.
So, Craig argues:
P1: If God did not exist, then objective moral values and duties would not exist. 
P2: Objective moral values and duties do exist.
C: God exists.
Let's define 'c-god' as an essentially maximally cruel agent.
P1.3: If no c-god existed, then objective cruelty would not exist.
P2.3: Objective cruelty does exist.
C.3: At least one c-god does exist.
And so, there exists at least one essentially maximally cruel agent – who would be the paradigm of cruelty.
So, let's consider some questions:
1. Is cruelty, objective, in the relevant sense of 'objective', that is, the sense in which Craig uses the word 'objective' in the context of his metaethical argument?
Yes, since it meets Craig's conditions for objectivity.
In fact, one may mirror some of Craig's own arguments on the matter of objective morality here:  The Holocaust was cruel, and it would have been cruel even if the Nazis had won the war and convinced every human person that it wasn't, after killing all dissenters, brainwashing children, etc.
Moreover, whether a person is a cruel person, or whether an action is a cruel action, are matters of fact, not matters of taste or opinion.
2. Is cruelty just the absence of kindness, so that – perhaps – there is no ontological foundation of cruelty, but just an ontological foundation of kindness?
No, cruelty is not merely the absence of kindness.
To see this, let A and B could both have no kindness whatsoever, as follows:
A is completely indifferent to the suffering of others – or, indeed, to anything that happens to others. For instance, if A sees a man torturing and killing children for pleasure and A can stop that easily and effortlessly, A feels no motivation whatsoever to do so, and lets it happen. On the other hand, A has no interest whatsoever in inflicting any pain or suffering, either. A might inflict pain, etc., in self-defense if needed – for example -, but only as a means to an end – i. e., to defend himself -, not because he cares whether someone else suffers. A simply doesn't care about the suffering, happiness, etc., of other agents.
B has no kindness whatsoever, either. However, B revels in horribly torturing other people just for pleasure. So, B is surely extremely cruel.
It might be debated whether A is cruel, but even if A is cruel, surely B is more cruel than A.
Yet, if cruelty were merely the absence of kindness, two agents that are equal with respect to kindness would be also equal with respect to cruelty, and A and B are equal with respect to kindness – i. e., they both have none at all -, but not equal with respect to cruelty.
Hence, it is not the case that cruelty is merely the absence of kindness.
3. Can objective cruelty exist without God?
It can exist without God – of course I hold that does, though theists do not know that -, but here I'd like to point out that Craig himself holds that if God did not exist, rape would not be morally wrong – he is mistaken about that -, but would still be cruel – he's right about that.
So, the ontological foundation of cruelty – whatever that is -, is not God, or something whose ontological foundation is God, etc.
Given that defenders of Craig's argument often demand that non-theists who reject the first premise of his metaethical argument explain what the ontological foundation of moral values and duties would be if God did not exist, one may – as an alternative or in addition to arguing that the demand is improper and non-theists who reject the first premise have no such burden – mirror the argument, and ask that theists who reject the first premise of the metacruelty argument for the existence of an essentially maximally cruel being – a c-god - explain what the ontological foundation of objective cruelty would be, without a c-god.
There are some replies available to the defender of the metaethical argument, and it's beyond the scope of this brief post to analyze them, so here I'll just say that in my assessment, a non-theist has at least no less plausible mirror responses for objective moral values and duties, but I just wanted to briefly raise the issue (I will analyze a couple of potential theistic objections in the longer reply I'm writing).