Those 2 deserve to be tortured for what they did to that poor man...

Well this is how I took it...They got the wrong guy because the real killer got away. He wrote that note so that the parents could find peace. If they found out that they'd got the wrong man after all that, then they'd feel even worse than they already do.
Who knows this is just my opinion!

Actually, the tax man really had amnesia and believed he was actually the killer. So he wrote the note because he felt remorseful. He did not remember anything. When they tortured him to speak the son's name, he was able to say the name correctly because he overheard them mention Benjamin. The flashback hinted at it.

He had amnesia. He didn't remember being in that van with a convicted child killer, so from his perspective, he had been wearing a prison jumpsuit, so he must have done something wrong, right? He had come to believe what the couple was telling him that he had done. He was NOT the killer; they tortured the wrong man. The only thing that doesn't make sense to me is how he knew Benjamin's name. To those saying that they said it while torturing him, watch the film again; they did not.

That's true, but if I remember correctly, it was just tax evasion or something like that. It wasn't violent and certainly wasn't something that warranted being tortured like that.