The meaning and relevance of this movie's 'The Villain' and 'Cactus Jack' titles are that they both refer to the name of the film's central antagonist character of Cactus Jack Slade played by Kirk Douglas.
Cactus Jack Slade, the character played by Kirk Douglas, is a human character form of the Wily E. Coyote character from the Warner Brothers 'Looney Tunes' / 'Road Runner' cartoons. This film though does not feature a human-form equivalent of the Road Runner character.
This movie's original American title was changed from 'The Villain' to 'Cactus Jack' for release in foreign territories such as the UK and Australia after the picture failed commercially at the box-office in the USA.
The central character in this western-spoof is Cactus Jack Slade. Similarly, there had been another western-spoof with a central character called Evil Roy Slade (1972). This was a tele-movie that had been made earlier in the decade. Both the Evil Roy Slade and Cactus Jack Slade characters are seen dressed very much alike, completely in black outfit with black hats, both sport moustaches, are villains, and share the same last name. The Slade surname in the earlier film was an acronym for "Sneaking, Lying, Arrogance, Dirtiness and Evil." Moreover, both characters in posters are seen riding atop a Western vehicle, Jack on a locomotive and Roy on a horse carriage.