The movie is based on "Diabolik", one of the longest running - and most successful - Italian comic strips. It was created by Angela Giussani and Luciana Giussani, two Milan sisters who built a small and very profitable publishing empire out of the "King of Terror"'s success. In the paper version, "Diabolik" is much more sinister than its cinematic counterpart - he's a criminal fighting evil with evil, often resorting to murder to "punish" the evildoers he meets. The movie was made assuming some knowledge of the comic strip, thus explaining the negative reaction it gets outside Italy.
This film was produced by Dino De Laurentiis (later responsible for the infamously mammoth King Kong (1976) and Hurricane (1979)) and its #3-million budget was much more than director Mario Bava was used to and enabled him to work with more money and a much more prestigious cast than he normally did. Howeer, he remained true to his principles, relying on imagination rather than money, and brought in the film substantially under budget, at a mere $400,000. De Laurentiis was so impressed with Bava's saving him so much money that he offered Bava the opportunity to make a sequel with the leftover funds, but Bava had by then tired of working with the producer and decided to pass.
According to John Phillip Law, the first choice for the role of Ava Kent was an unknown model from New York, who was a friend of Charlie Bluhdorn, the CEO of Gulf & Western, the conglomerate that owned Paramount Pictures (which had contracted to distribute the film in the US). Second choice was Catherine Deneuve, who lasted for about a week, until director Mario Bava decided she just was not right for the part. He finally found the perfect Ava Kent in Marisa Mell.