It WAS the killer they were torturing. I'll explain.
Many people think that the couple had gotten the wrong man when it's revealed that there were two convicts, but that's wrong. The whole thing is actually meant to a play on the audience's psyche. Throughout the whole movie, we think that the couple is getting their revenge on their son, and as sick as the torture is, we find it justified. At the very end, it reveals that there were 2 convicts, and we're meant to believe that it was a twist and that the couple had made a mistake, making us horrified at how we thought of the situation. We're meant to be shocked and appalled at how we as an audience were supporting the couple, only to find out that it wasn't the killer, since throughout all the pain we keep thinking "he deserves it", only to find out that we were wrong. But the suicide note shows us that he WAS the killer all along, and we were just meant to think that it was a mistake, allowing us to breath a sigh of relief that the wrong man wasn't tortured after all. What we thought was a twist, wasn't actually a twist, just something to make us panic.
I actually saw something like this in another movie, where the same thing happened, only it actually was a innocent man. This movie did something similar, but didn't follow through with the twist, revealing it to be an attempt to fool the audience. The point is to also make us wonder who the real monsters were when the "justified" torture was questioned at the possibility that they got the wrong man. Even after it's revealed that it was the killer they were torturing, we're still left with the uneasy feeling of the possibility that it could have been a innocent man they had tortured, though it also gives us a feeling of relief that it wasn't. That's the beauty and creativity of the film that many people seem to have missed.