Twice Dying by Neil McMahon

Outside San Francisco, a sting of NGI's--violent sociopaths found not guilty by reason of insanity--have been sent back into society, and, one by one, mysteriously disappeared....

Beautiful psychologist Alison Chapley wants to know why and sets off on a search for answers that lures her and her former lover, medical examiner Carroll Monks, into a menacing and increasingly macabre dance with a brilliant psychopath known only as Naia....

Naia's perverse and carefully chosen clues--plaster masks, a snuff videotape, a deadly cobra--draw Alison into a psychological seduction she's powerless to resist. Running out of time, Monks's only chance to save Alison is to outwit this twisted mastermind in a deranged and deadly game of cat-and-mouse.