Since John got away, and saw what was happening in the house, saw the parents, knew what was going on---couldn't he tell the police that's how it all went down and how he "escaped" to make himself look better and get the parents in trouble? I feel like he would have told them that, and at first they may not believe him but if they found that house--their DNA is all over it. And they'd totally be caught.
shareWell maybe he was relieved he was off the hook. Getting involved could bring the focus back to him maybe?
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Agreed with PenguinCat. Also at that point he genuinely might not have even cared about getting the parents in trouble. Why would he? If anything it gave him more time to get away and escape - he might have figured the cops would find them and as long as they were off his trail, that would be fine with him.
Although one wonders if the police might do any investigating at the house, and find any evidence, on their own...