Viral meningitis.
Cushing's disease.
Psychosis.
Certain diseases that affect the brain can, in rare cases, dramatically alter an individuals personality. What this tells me is that much of our will is directly controlled by subconscious processes we cannot control. Reading the works of Oliver Sacks has led me to believe that while we have some free will, or can make some reasoned choices, the data that we rely on to make our choices is beyond our control.
There was once a time I could not enter my home alone without going through a long involved process to ensure that there wasn't someone hiding in the house. I would enter a specific door in a specific manner, jump over to the fireplace and grab a poker and then proceed a systematic check of every room, closet, closing doors behind me, proceed to the basement, etc. Rather annoying. This form of high anxiety borderlining paranoia would exhibit in other situations as well. Then I started drinking and using drugs. I noticed a remarkable difference in the way I handled certain situations. I no longer exhibited the anxiety when alone but the paranoia in other situations would still manifest.
Then I received treatment with Lithium. There was a remarkable change in the level of anxiety and how I handled certain situations.
All due to chemical reactions within my brain introduced by chemicals from the outside.
I see some free will in the manner in which I acted on certain situations but no free will in the underlying thought processes that gave rise to the possibility of options. At least, that's what I like to think.
I'm not making any sense.