Very interesting how Voldie does these killings to throw suspicion on Harry and Rufus. But Voldemort is too much of a psychopath to be so subtle. The FBI and other crime organizations have psychologists helping solve crimes precisely because criminals have a dastardly difficult time not following a set pattern, no matter how intelligent they are. And Riddle is one of the biggest egotists around, certainly he is incapable of killing or ordering a killing without the dark mark involved. That's also the reason why he has no unmarked followers. There may be unmarked sympathizers, but there are no unmarked Death Eaters. If someone is a part of Voldie's camp, he/she has the mark.
This is a great read, and I'm really enjoying it.