There are a number of potential problems with the free will defense (FWD) to the problem of evil and its many varieties. Here is one very simple potential problem.
Let me begin with a quick story. I have a young son. We have an open stairway leading into a finished lower level. We have a gate blocking this open stairway for safety.
Now consider the story told by many Christian philosophers, including Alvin Plantinga and Peter van Inwagen. In response to the logical problem of evil (i.e., whether there is an impossible-to-overcome inconsistency between the existence of God and the existence of evil), they will claim that at least some of the evil might be attributed to free will. After all, God can only do what is self-consistent, and they say it would not be self-consistent for God to both (a) allow creatures to have free will; and (b) ensure those creatures only made God’s preferred choices.
Now, I think we can allow this point, but still maintain there is a problem. There is a gap between making choices and real world undesirable outcomes. Consider back to the example of my son. He is allowed to try to open the gate, go down the stairs, etc. In other words, his choice is perfectly available to him. Yet, I (in my all-knowing, wise, and loving epistemic state) know that this choice could potentially bring about terrible consequences. So, I prevent the outcome without preventing the choice. It’s not at all clear why God is not in a similar position. “Bad” people can make free choices to try and kill someone. Allowing them to be free doesn’t include the requirement that they succeed. There is precedent for this. As I’ve discussed before, I have the ability to try and will myself to jump into outer space, but I am physically not able. This is not considered a violation of my free will. Or I could try and shoot someone, but they may have a bullet-proof vest that prevents the bullet from harming them. Again, this is not a violation of my free will. This scenario still allows me to make free choices, which people often call a greater good, and it even still allows God to judge me based on my choices, if that’s what your religion says will happen. Imagine, there could have been no Holocaust, no violation of free will, and Hitler would still end up in Hell for being a rotten son of a bitch.
So, here is one way in which the FWD may not be a good solution to even the strictest form of the problem of evil.