I actually think I've thought up a pretty good theory on this guy and am positive that you are all going to be awed by my level of super genius. Well anyways, I'm thinking that Arthur is the killer, and not only that, but he possesses powers of possession (lol, sounds funny the way that came out), and that's how he would be killing everyone.
Now think back to the photograph of the twelve Company Leaders, and notice how only the right side of his face is visible to us viewers. What possible reason is there for this? Why exactly did the Heroes Production team go out of their way to have the actor be facing that direction as the photograph was taken? Could it possibly be to hide an extremely hideous burn, somewhat similar to that which Nathan keeps on seeing on the left side of his face (same side that can't be seen of Arthur in the photo), so as not to spoil a major storyline? Am I getting onto something here?
Now think back to when both Kaito and Angela receive their death marks. Who was around both times? That's right, Nathan was! We see him knock into Ando just before Ando delivers Kaito his paper, where the Death Mark is found, and he only just enters the room in which Angela is, whereupon she leaves and stumbles onto a death mark of her own? Coincidence? Or is dear old Arthur using his son to in part do his dirty work? It would certainly fully explain this burn that Nathan keeps on seeing on his face as well (that it would seem likely that if Arthur can possess people, a part of him can be seen on who he possesses: his burn).
Now let's look back to Kaito's death. I'm guessing that Arthur yet again used his son Nathan to do his dirty work. Now you may ask why he decided to use Nathan to kill Kaito rather than directly possess Kaito and kill him himself. Well firstly, consider the implications that Kaito and Angela had at some time in the past had an affair, and that Arthur had known. Could he, in fact, have wanted Kaito to know that he was the killer (which would be apparent if the burn does indeed appear on whoever he's possessing)? Might he have actually wanted to kill him with his bare hands rather than with his mind to really enjoy the kill? Plus, he would have known that Kaito's power would have left him pretty much defenceless in his position (as they clearly did), and that with Nathan's ability, he would have been able to pull it off (as he could have just pushed him off and flew away at Mach speed). The fact that the scene just before shows Nathan looking in the mirror and seeing the burn on his face would certainly support this, and when Kaito claims that out of all of them, the mystery man was someone he would have never expected, well, that could simply be because Arthur was supposedly dead.
Now think back to when Angela appears to have been attacked by the killer. You might ask how exactly she was able to defend herself against such a mental attack, if Arthur can indeed possess people. Well, consider that not only do we not know what her power is (thus, her power could certainly be one that could act as a valid mental defence), we've also already seen her resist a mental based power in the show (Matt's telepathy). Though, she could simply have extraordinary strength of mind, and while struggling to resist Arthur's powers, she wasn't able to properly control her own power, which lead to the lights blanking out (scenario I think is more likely). Either way however, she appears to be able to defend against the power, which would explain how she was able o resist his powers if he can indeed possess people.
It's also quite funny that Nathan was around the very time Bob's death mark first surfaces. Another coincidence?
So yes, I'm thinking that not only is Arthur the killer, but he's using his powers of possession (which is in some way linked with his burn) to either control people (namely Nathan. lolz, alliteration, how smart a I?), or possess and kill them himself. Now bow down before me my below-genius-level-of-intellect bitches!