Spoiler** Innocent or Legally Not Responsible
I don't know how folks can say that Alex was innocent of any wrongdoing. We all agree that he knew that his aunt were holding those girls. He knew that he lied to police about not seeing them. He knew that harm would come to them if they were not found in time. He refused to answer questions when pleaded to by Keller and Nancy. He tried to run when cornered by police and when released by Nancy. He knew he had done something wrong.
Even if we can say that he had some mental deficiency, he knew the difference between right and wrong. Yet he still chose to allow the children's parents to suffer and to withhold what could have been life-saving information. Whether it was because he feared his aunt or had some type of need to protect her, he must have known what she was doing was wrong. To be complicit in her wrongdoing belies any conclusion of innocence on his part.
Now whether he would be considered legally not guilty because of his mental retardation is a different matter. But he is certainly not "innocent" in any sense of the word. And while I don't condone Alex's torture or any kind kind of vigilantism in general, I can certainly understand why Keller did it, especially in what he considered a life-or-death situation where time was of the essence.
But if Alex had no idea where the girls were, or if he was physically unable to verbalize where they were, or if he had been truthful to the police then I could agree that he was completely innocent and Keller should get the maximum prison sentence. However, if Keller survives that pit, I think only a light prison sentence would be appropriate. And if I were on that jury, I would be hard pressed to convict him at all due to the extenuating circumstances. I would figure that he had suffered enough.