If there are any doubts about this movie being an homage to the great American cartoons, the characters names here "Avery Simpson" and "Parody Jones" are clearly a salute to Tex Avery and Chuck Jones, two of the geniuses behind the Warner Brothers cartoon studio.
"Cactus" Jack Slade (Kirk Douglas) was a human character form of Wily E. Coyote from the Warner Brothers "Looney Tunes" and "Road Runner" cartoons. This film though does not feature a human-form equivalent of the Road Runner though.
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 U.S.
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 television movie. The Evil Roy Slade and "Cactus" Jack Slade characters were seen dressed very much alike, completely in black outfits 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." Also, both characters in posters are seen riding atop a Western vehicle, Jack on a locomotive, and Roy on a horse carriage.
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.