I can explain all that to you right now, without knowing how the show ends, and without any secret information.
- The antlers are a crown fashioned out of nature. A sign of royalty bestowed upon the sacrificial victims. It's a twisted mix of old pagan traditions and influenced by Madri Gras celebrations brought to the South by the French.
- Black Stars, Carcosa, and the Yellow King are all references to the brand of occult insanity the killer(s) subscribes to. It's no different from serial killers influenced by Lovecraft or satanism. It's an existing occult mythology from old fiction.
- The devil traps are another sign of how the killer(s) are influence by a mix of different cultures and mythologies which exist in the South. In this case it would be voodoo magic and similar sort of traditions.
They don't have to "mean" anything in the end. They're just very obvious icons which represent suppressed cultures and mythologies which exist in the setting, but are generally ignored and shunned by the larger Christian community. Old religions, pagan practices, etc.