Sometimes Lady Luck was smiling down on you and at the moment it was Detective Pete Sheridan's time. The suspect was ideal, exactly what he needed to get away with the murders. And it wasn't as if anybody would miss the guy, just some drifter who had killed before and had probably done other bad things as well. And he would make sure that Dean Winchester would not get the insanity deal he obviously had planned for emergencies.
Everything was going perfectly, he had the money, Diana and nobody was alive anymore who could link him back to the killings.