Was he? Well shit looks like I need to pay more attention, in that case it must have been obvious that they were about to perform a sacrifice.
Errol being the killer? I doubt that's a red herring since Nic Pizzalato said he's not interested in trying to trick the audience, I mean leading the audience to believe Errol is the killer and than revealing that it was actually someone else, it seems like it would be doing just that.
Plus on the inside video towards the end when Marty and Rust are on the boat, Nic mentions that we get our definitive look at our killer.
It's not the first red herring. And it won't be the last.