According to the Wikipedia entry of the Monster with 21 Faces (also named as "Fiend" in some sources), it started just after the very publicized Glico Morinaga case and it
... captured the Japanese public's imagination and many commentators refer to this incident as a turning point in Japanese society.
Information on the matter is scarce - at least in the western world. Was this similar to a Jack the Ripper / Zodiac Killer / Son of Sam case where mass hysteria followed alongside concurrent headlines by the media?