I wanted to see their reaction to finding out! (Spoilers)
So by the end of the story, they read the note and are happy that he's dead and "admits" fault. They got away with their revenge plan. They got their closure, and they get to walk away blissfully ignorant of the fact that they got the wrong guy.
But surely that won't last long, because it will be all over the news soon enough that their son's killer had been found alive and well, roaming the woods. Even if they happen to avoid the news, I'm sure the police would call them to inform them of his capture, just as he called to inform them that he had "escaped".
I was hoping we'd get to see their reaction to finding out that they tortured the wrong man! Instead they just drive off into the sunset. But surely they must find out eventually right??? Thoughts?