The Snorkel (1958)

Starring Peter Van Eyck, Betta St. John, Mandy Miller, Gregoire Aslan, William Franklyn
Directed by Guy Green
(actor & director credits courtesy IMDB.com)

A clever sociopath murders his wife and makes it look like a suicide, fooling the police, but not his stepdaughter, who's convinced of his guilt, and tries to find a way to prove it.

Van Eyck makes a smooth villain in this suspense thriller from Britain's Hammer Films, and the filmmakers hook viewers early with the intricately planned and silently photographed murder, in which the killer uses a snorkel to breathe the harmless outside air while his wife asphyxiates on vapors from the gas lamps he turns on.  From there the race is on to see if the stepdaughter can find the evidence to expose him, or if he'll knock her off first, setting the stage for the finale in which one of them overplays their hand.  Although the rest of the film doesn't quite deliver as well as the opening scene does, this is enjoyable fare, with a story that avoids using the more relentless twists to be found in Hammer's thrillers to come.