To be fair, he didn't kill anyone. He was riding in the car with someone, who decided to stop at a "friend's house", and that person killed that friend. It was a drug deal gone bad, and Dog just happened to be along for the ride. His conviction was for accessory to murder, for being present at the scene of the crime. He did his time, and was released. During his time in prison, he decided to make a change for the better in his life.
I believe that simply the fact that he's not perfect, and has gone through hell and done something with his life, makes him interesting to watch. He's literally been there with the people he's catching, unlike most bounty hunters. He knows what it's like to have to go to prison.