Evil, in a manner of speaking. In reality, most who have been described as 'Demonic' are usually just lonely and in need of love. Or it could just be that they are 7 ft tall, wear a cape and have glowing red eyes. And eat people.
When an inanimate object, such as a film or potted plant is described as demonic, the term often means 'evil, dark as hell, or sickly twisted'. This is only in context though. It could be you are referring to the pot planbt of a Mr. Bee El Zebub.
"That programme was demonic! Especially the part with the shredder in the cats home..."
"See that kid? The one who's setting fire to that old lady? Yeah, the one with the tail and hooves. Demonic, isn't he?"