It's just a matter of which person is Caspere's kid.

The father/son stuff is very obviously the focus. You didn't even mention Vaughn's big speech about how his father fucked him up, the obvious ways in which Farrell's dad fucked him up, etc., so I'd be stunned if it's not Caspere's illegitimate kid who did it. I mean, they spelled it CasPERE, for fuck's sake. That can't be coincidence.

It looks like the show *will* be moving forward into the future, so I wouldn't be surprised if we continue to see how fathers fuck up their sons.

I did read an interesting theory that the killer could be Frank, that he had a split from reality during the traumas of his childhood, and that he could be unaware of what his other self is doing. That may be a little cliche for my taste (the old "the detective is the murderer!" thing), but I trust Pizzolatto, frankly.