Hong Kong action filmmaker Dante Lam (“The Beast Stalker”) has made a spooky if overly complicated thriller in “That Demon Within.”

Though full of wild-eyed effects, creepy environments and subplots involving stolen diamonds and repressed memories, this tale of madness and graphic violence might have been more interesting with a tighter focus on a key relationship between the story’s cop hero, Dave (Daniel Wu), and a freelance bad guy, Hon (Nick Cheung).

Dave inadvertently creates a bond with Hon by donating blood to the injured crook, without knowing Hon’s identity. In several subsequent scenes, Hon seems to appear out of nowhere to give Dave assistance at critical moments, taking out villains in gruesome ways.

The delicious suggestion is that Dave and Hon are linked by the sharing of a life force and, more darkly, a killer instinct.

Dave, who at first blush seems a starchy law-and-order man, is actually sitting atop volcanic emotions and a secret history that somehow involves fire. The meaning of his grisly flashbacks and tortured recollections become shockingly clear in an apocalyptic ending.

But is Hon actually there, or is he a product of Dave’s feverish imagination and growing taste for vengeance? That Stephen King-like angle is the richest material in “That Demon Within,” which is otherwise another tale of gangsters turning against one another.

Tom Keogh: tomwkeogh@yahoo.com