So, they want an Aquaman-related horror movie instead of a random horror movie?! Why not?
Similar things have been done before. After '40s horror star Rondo Hatton played a villain called the Creeper in a 1944 Sherlock Holmes movie, Universal tried to spin him off into an independent Creeper film series (unconnected to the Holmes film since his character had died, but otherwise basically the same character), although he died after only two films had been made.
And it's not like the DC Universe hasn't always had a horror side -- Swamp Thing, the House of Mystery, Constantine, Lucifer, etc.