Yup. In my opinion/experience, this is a very REAL thing. Not just some "religious whack-O"s idea, nor somebody's "invisible friend", nor some mental illness, nor chemical imbalance, psychosis, etc.
Believe what you want, but demons are real, demonic influence on people is very real, and observable in many cases. Sometimes/often masked as a medical/mental health issue.
People who deny the existence of unseen spirits are, in my opinion and experience (I am a Pentecostal preacher) absolutely ignorant of the hidden reality of the spirit world. I can't prove the spirit world exists, and you can't prove it doesn't. But guys/people like the subject seem to support my (correction, the bible's) assertion that evil spirits CAN and DO influence people to do bad things.
Can't tell me it doesn't exist... Seen it too many times.