“Can a brain tumor cause someone to depart from an otherwise normal life and go on such a horrific killing spree with no outward warning signs?”
We don’t know that there were no warning signs. Aggressive. brain tumors can develop rapidly. Someone acquainted with him might have noticed changes in behavior and affect if a neurological condition was developing.
Paranoia might be symptomatic ... last residence was very isolated.Wonder if his earlier residences [not his rental properties] were also isolated?