Thank you for your question regarding the problem of evil and, specifically, the problem of horrendous evils. I can see that you have thought deeply about and are already familiar with the works of a number of the relevant philosophers of religion in this field.
As you say, the ‘free-will defence’ might be able to speak to the question of why God would allow evil and suffering in this world, generally speaking; but as you ask, what about certain specific instances of evil and suffering, horrendous evils of the type mentioned by Marilyn Adams which you refer to in your question, the sort of evils for which we simply cannot conceive of any possible good reason that God would allow such a thing to happen? Do these evils give us reason to doubt that a good God exists?
I remember thinking about this question when I took my daughter Grace to the hospital when she was a one year old to get her immunization needle. I can recall nervously wondering what she would think of me when the nurse stuck the needle in her arm as she sat on my lap and realized that I was actually allowing this to happen to her. Would she think I didn’t love her anymore? Would she think I was wanting to hurt her?
Well, when the needle went in she immediately started crying, and as she looked at me through tears of pain the burning question on her face was “why?”. “Why daddy are you allowing this to happen?”
Thankfully, I never once sensed that she thought that this meant that I didn’t love her anymore. Rather, if she had been able to put her question into words I believe it would have been something like this: “given that I know that you love me, why daddy are you allowing this to happen to me?”
And the fact was, as much as I wanted to explain to her the reasons “why”, she just didn’t have the capacity as a one year old to understand them. One day she would possess the capacity to be able to understand. But for now all she had was trust.
Likewise, as a child of God I’ve learned to trust that God does have good reasons for all that he allows even if I can’t understand all of them now. One day I will understand. But not today and in all likelihood, not in this life. Someone may object and say that this is just blind faith. But it’s not. It’s a faith based on the evidence of God’s love for us, just as Grace’s continued faith in me, in the midst of inexplicable pain, was based on the evidence of my love for her from the very first day of her life and throughout her life again and again and again.
Compared to God, we too are children. We are finite. We don’t know everything, but we can have faith in the one who does. Not only because his ways are higher than our ways but also, as the Gospels make clear, because God is not aloof from our suffering.
As the Russian novelist Dostoyevsky realized, with awe and wonder, as he stared at a painting of Jesus’ body: “no other God has scars”. We may not know the exact reason why we suffer in any given instance, but one thing our suffering cannot mean. In light of the cross it cannot mean that God doesn’t love us.
In other words, on the question of suffering, we have some answers, better and more nuanced answers than any other worldview is capable of offering. But when it comes to specific incidences of suffering we don’t have all the answers. We can say some things about why God could allow suffering generally. But we cannot speak to each and every particular instance of suffering and say I know why God would have allowed that or that or that to happen.
But we shouldn’t expect to know. Because we are finite. That is part of what God’s answer to Job at the end of the book teaches us. That we cannot expect to know everything because we are not God. But He does know everything and his purposes are good. So it doesn’t make sense to judge God just because we can’t make complete sense of it all. We have more than enough reason however to trust the One who can make complete sense of it all, our heavenly Father, the one who “spared not his own Son but gave Him up for us all”.