Now I'm thinking it's some wacked out teenager or young adult.
I think somebody really screwed up by leaking the Tarot information, but it could benefit the investigation because I don't think the killer is finished yet. Now that the Tarot card is public, depending on what kind of mentality it is we're dealing with, the next time (and I'm convinced there will be a next time) the killer might make a more flamboyant "statement". The more flamboyant he gets, the more chance for clues to be left behind and as the killer "acknowledges" the publicity, the more likely he'll be to start making real mistakes. If the killer is a teenager or young adult, the pressure of law enforcement chasing him will start to get to him sooner. A teenager or young adult will crack sooner than an older, more disciplined person.
I'm more inclined to think that any benefit would be along the lines of spotlighting this killer to any acquaintances who would know about an association with Tarot cards. Someone out there knows or suspects who this killer is. The information about the Tarot card may help convince someone to tip off the police. At least I hope there would be a bright side to this leak of info.