If God permits evil to protect free will, God cannot be exclusively good.
For God to be benevolent and exclusively good, nothing can be more good than God. If we can imagine an alternative divine being that did not permit evil, even if it meant sacrificing an element of free will, then this supreme being would be “more good” than God.
Because we can contemplate another supreme being “more good” than God, God cannot be the “most good” divine being we can imagine. Therefore, God cannot be all good, and therefore, cannot be benevolent.
This leaves a situation where either God is not exclusively good or does not exist. Either one is catastrophic for our understanding of God.
Additionally, while the preservation of free will above all else adequately explains evil as a result of human behaviour, it cannot account for evils that lie beyond our control. For example, childhood cancer is not the result of human free will. It could be eradicated without infringing on our free will.