I think it's the Sheriff. First, I believe that the guy who shot Archie's dad at Pops Diner was just a random robber because he acted way too anxious after he saw that there was another person in the diner and ended up shooting Archie's Dad when Archie tried to intervene. At some point the Sheriff gets involved and for whatever reason (I am guessing he doesn't like all of the crazy stuff happening in town), assumes the identity of the robber and goes around killing people. Later on in the season, I think Betty's brother is going to show up and a lot of people are going to assume it's him. Hell, even right now they are somewhat pushing the idea that Betty could be the killer in some odd dual personality kind of way, or it could be her father, but I am sticking to my guess that it is the Sheriff. We just need to see if anything happens to Kevin because that would help determine if the Sheriff possibly is the killer.