Just because people with psychotic issues report issues with demons proves nothing. Many people without psychotic personalities also claim to have had issues. It's like people who have had near death experiences and people who see UFOs. Certainly there are some fruitcakes out there, but there also are good, decent, honorable people who say they've had experiences with the paranormal. (Even psychotics have normal experiences, so just because some psychotics say they saw something they can't explain doesn't cover all the others.) People all over Phoenix, Arizona, saw UFOs (some as large as football fields) on March 13, 1997. How does one explain that?
Atheists have to discount the paranormal world of ghosts, spirits because of the implications. They thus keep their minds closed to the possibility that there's life after death, or beings of evil. I think atheists can be agnostics when it comes to the paranormal. The existence of life after death can be argued that there's a God, but it doesn't necessarily mean there is a God. Perhaps there's life after death but no God. Or perhaps we exist for a while, then cease to exist. But if God exists, and He doesn't write His name across the heavens to prove He exists, there's a chance that evil spirits aren't under the same restrictions. From what little is surmised from the existence of spirits, they don't seem to be anywhere near all powerful; they also apparently are not permitted or able to take human life unless people give their lives totally over to them.
Whatever the explanation, I think people are seeing and hearing something they can't explain. I don't believe all the videos and photographs are faked. People have had stories of hauntings in all cultures and times going back to ancient Greece and before. And the invention of cameras and film haven't seemed to have had any effect on the numbers of claimed hauntings. It leads me to wonder that if an atheist were to experience a haunting first hand, would it necessarily convert them to believing in a God?