It's when innocence is allowed to be corrupted that it doesn't seem all that fair. Someone brought up Job for example but Job was not innocent and willing to do anything for God but even worse things sometimes happen to innocent children. The OP mentions a movie where a child is kidnapped and brutally raped and murdered, the kind of stuff that has people crying over movies, but as the movie got into why, it showed the perpetrator going through horrible stuff as a child themselves meaning they only further progressed the corruption of the father and his father and so on.
Ok so a person has been abused, grows up and goes onto be abuser themselves. I'm pretty sure, such a person would have been able to keep their unbalanced views from getting attention, allowing the to get close to their victim. This shows they had control and rather than get therapy and help, they instead, used their free will, using what happened to them as an excuse to inflict harm unto others. Such a person is fully accountable for their actions, and if they get away with it in this life, Judgement still awaits in the hereafter.
Now if the same person had a mental illness and was not in control of their actions, then they are not considered accountable in the sight of GOD. He will be fully aware of their mental state, as nothing is hidden from Him.
Ultimately free will means we all are given choices, just as Adam and Eve, pbut, (peace be upon them) were given. They realised their mistake, and turned to GOD in repentance. Whereas Satan, (Iblis) refused to repent, rather blaming his downfall on others. 2 choices, 2 very different paths.