"Citizen X" tells the story of "The Butcher of Rostov", nickname for a heinous and perverse Russian serial killer who claimed 52 lives from 1978-92. The film focuses on the novice detective (Rea) who doggedly pursued the killer against all odds in the face of an uncooperative bureaucracy in self-serving and convenient denial. An HBO product for t.v., the film offers a solid cast, good performances, spares the audience much of the grisly details, but plays out like a docudrama sans the stylistics of similar Hollywood fare. An even and straight-forward dramatization of a serious and comparatively little known story more interesting than "Jack the Ripper". (B)