P.S.: Shouldn't it be The Threatened (Menaced) Murderer, rather than The Threatening Murderer?
Neither Les Vampires nor Fantômas is currently available on video or laserdisc in the US. However, Water Bearer Films (of New York City) apparently plans a US release of the restored Les Vampires on home video as a deluxe boxed set next October. Les Vampires elaborates on may of the elements of the Fantômas novels and films, and is Feuillade's masterpiece. I can't recommend it highly enough. Watch for it, and lobby Water Bearer Films to follow it with Feuillade's other serials: Fantômas, Judex, and Tih Mihn. Many thanks to Tim Lucas for the news.
As for the appropriate translation of the title of Rene Magritte's painting, it is generally rendered as The Threatened Murderer. However, a comparison of the painting with its source (a scene from Feuillade's third Fantômas film, Le Mort qui tue, shows that the men lurking on either side of the doorway are most likely Fantômas' henchmen (i. e. threatening murderers), not detectives. You can see the painting and a film still at Fantômas & The Avant-Garde for your own comparison.