I’ll grant you I’m a bit rusty on demonology and i didnt go to medical school, but I do know a tiny bit about brain tumors. A loving and kind uncle became a raging monster over a period of about four months. Luckily, if that’s the right word, he had a mild heart attack and a complete work up led the docs to discover the tumor. He recovered and, while he lost a bit of mental acuity, he was his old self again. My aunt was the saint in all this sticking with him.
It looks like if it were that common they would give a Cat Scan to every criminal to check for tumors. Maybe with an operation, we could clear out most of our jails and send them home. Most people I've seen however, that have a "mean streak" just listen to the voice in their head that tells them to trust no one, nothing wrong with me, you're the problem, I know how to fix the problem, BANG! If you read the 4 Gospels, Jesus spent much of His time casting demons out. Most Christians seem to skip over those parts and think it doesn't happen today. When you say "demons" people get uncomfortable and want to skip on by. Perhaps a better wording might be evil spirit. Satan's job is to steal, kill, and destroy. He can hide a demon in there with you and you think it's you telling yourself to cut your wife up in bits and bury her in the yard. He can convince you it's the right thing to do even though you may not even be able to kill a chicken to eat. Drug addicts almost always have personal problems, looking for pity, crying out for help, and take drugs to hurt the people that love them. That voice tells them to take more and more till someone pays attention. Many times they hurt themselves to hurt the person that hurt them. Alcoholics= same thing.
Jesus Christ can come in, if invited, to clear out the evil and replace it with the Holy Spirit, to comfort you and console you, build you up, and teach you. That is basically the 12 step program. You have to want help, realize you can't do it on your own, and ask for a higher power to help you. Our flesh is weak and the devil knows it. It is a war going on in the spirit world and we are just pawns stuck between good and evil. As much as we try, usually we listen to evil and shun good. We are so weak without God.
I'm sure it could be a tumor also. I just haven't seen any.