If you believe in God and the stories depicted in the Bible, then you believe in demons. If you don't believe in demons, then there is some doubt it you on the authenticity of the Bible - that is completely fine, we all struggle in different areas. However, evidence of demons is most well supported in research of pagan cultures, and even modern demonology today.
More or less, ancient "pagan" cultures all had some form of contacting and worshiping spirits. All of them are named differently, of course, but they often share the same symbolism and ritualistic practice - all dating back from Nimrod, the symbolized sun god of Babylon, aka Lucifer. The Incas, Aztecs, and the Egyptians are renowned for their sorcery and human sacrifice. The parallels between all of these cultures is astonishing. Teachings from these ancient cultures are used by Luciferians today; likewise, people today are able to harness the supernatural powers granted to those who please Satan and his demons. Great power, but also grieving demise are upon these people today.
Are ALL proclaimed insane people today actually possessed by demons? No... It doesn't seem so.
Are they influenced by them? I would believe so...
Are SOME people insane because they are possessed by demons? Yes, if they allow it.
Is demon possession real aside from clinical insanity? Yes.
There is no bounds to the amount of evidence you can find online if you are willing to look into it far enough; however, I would not suggest it. Be assured: demonic existence, influence, and power to those who submit is a definite.