Who Did You Think The Killer Was? *Spoilers*

When I initially watched this, I kept thinking who the killer was going to turn out to be?(I think that's common for most in "whodunit" mysteries like this.) Anyways, even though they said they confirmed that the one guy whose body they never found,(Either Locus or Stephen, can't remember which), was his blood type found at the scene and that he would've bled to death trying to fake his own demise, I still thought it would've been him. I don't know how he would've done it; Maybe steal some of his blood type from a blood bank, IDK. But I thought the fact that his body was never found meant that he was in fact the killer, and he was still alive. The Narrator, David, was actually my second choice because of how creepy he came off, and it's who they ended up going with.

Your initial thought as to who the killer was? Were you right or wrong?