It would help to have more context, as this is a rather convoluted sentence with three different characters in it.
However, my understanding is that Brock's career at the Daily Globe was ruined, because he wrote a story that claimed he had discovered who the "Sin-Eater" was. However, the man he wrote about as the true "Sin-Eater" was NOT actually the true Sin-Eater, and instead was just a "compulsive confessor." This last phrase, "compulsive confessor," is meant to be funny. Usually we call people "compulsive liars," meaning they can't stop lying all the time. In this case, the author changes the word "liar" to "confessor," meaning a person who can't stop confessing all the time. In this case, this man confessed to Brock that he was the Sin-Eater, when he really wasn't, and later it was discovered that he was not the true Sin-Eater. That's why Brock's career was ruined, because he announced the wrong person in his news article.
Sorry, it's a long explanation, but hopefully that makes sense. :D