*obviously spoilers but just to be safe*
I don't think that it could be any of Dave's friends. The biggest problem I see on this theory is the fact that EVERY Dave's friend can die including Hoss. I also don't understand why a knife suddenly shows up in Hoss' chest in his bad ending. And THE SAME KNIFE that kills Oswin. (Tyson also says that he only dazed him, not that he used a knife)
For the culprit I have so many diffrent theories from ghosts to the test subjects of the experiment, Dave and Thanatos. So far I only include ghost stuff as something to point out because they can basically do whatever. I also am looking out for Thanatos because there is just something wrong with him I can't quite place. (Benson should not be the murderer, I don't see any motive and the next update should confirm it hopefully)
And the driver? I don't quite understand. Some of the BGs are oddly white on its sides, which I only assume means it's a flashback? I studied a bit the background pictures but no luck. Do you mean like the CGs or...?
I tried my best to write everything I know but I'm quite bad at doing theories. So what do you think?