Are the filmmakers trying to make some political statement at the end with the suicide note? People will confess to anything, even the most heinous acts imaginable (ie: torturing & killing a child), if they are tortured enough?
I assumed that the man simply felt sympathy for the couple despite what they had done to him. Losing a child in that way is so horrific, and he didn't want them to have to experience the pain of knowing that they had tortured an innocent man on top of it. He knew he was going to die either way, so why not just give them the closure they needed?
There was no need for him to confess to the crime if he was just going to kill himself. You would only do that to make the punishment stop.
A lot of people on here raise the point that he knew the kid's name when he shouldn't of done. They have tvs in prison (so maybe saw it on the news?) and people talk, so it's definately possible that he heard it from another inmate or maybe had a chat when in the back of the police van with the killer himself....