Sometimes videogame demons are very, very obvious, but other times, they're a lot more subtle.
It's said that the devil can wear many forms, and that's just as true in videogames as it is in theology. In issue 272 of The Escapist, Matthew Gault makes the case that while the stuff you're fighting might look like alien horrors or killer robots, they're actually just demons in disguise.
Open your mind enough, and you can see demons and angels going toe to toe in some surprising places, says [Tamir] Nadav [a designer on Warhammer 40,000: Dark Millennium Online] ." Halo actually is the concepts of angels and demons in a futuristic society. The angel would be Cortana. She represents a celestial being that's invincible and manifests only to the main character. The Covenant represents the demons, trying to conquer, trying to destroy, with evil plans of consuming all of humanity. They basically have all the traits we ascribe to demons without the red horns and pitchforks."
Your conscious mind might not immediately associate those Grunts and Brutes with imps and demons, but we've become so familiar with the concept of devils, your subconscious connects the dots for you, says Nadav. "While they may not be cookie-cutter to what you picture, that concept is still there and still strong. I'd venture that a lot of successful games outside of the casual markets on consoles and sports, do have those themes hidden inside them," he says.
The age-old struggle of Heaven and Hell makes a great basis for videogames, which is why it turns up in some many different guises. Gault suggests, however, that such a familiar theme might not always be the best one to pick. You can find out more by reading his article "The Devil and the Game Designer."