Olney murderer Ben Field would have without a doubt gone on to kill again, his victim's family had claimed.
Field, 28, who is a churchwarden and the son of Olney's former Baptist minister, is awaiting sentencing for killing 69-year-old scholar Peter Farquhar to inherit his money.
He duped Mr Farquhar into a fake gay relationship to persuade him to change his will. But then he slipped drugs into his food and drink until he became a "shambling wreck" of his former self, a 10 week court trial heard.
Finally, Field, who lives in The Manse in Olney, smothered the elderly gentleman with a pillow.
Now Mr Farquhar's brother Ian has told the BBC: "His brain was his gift and it was significant that was one of the things that Ben Field attacked.
"He attended stuff with my brother with the highest people in the land academically, and they all thought he was fine," said Ian Farquhar.
He said he had "no doubt at all" that Field would have targeted other victims if police had not caught him..
They began investigating after the death of Mr Farquhar's neighbour, 83-year-old spinster Ann Moore-Martin.
Field was in a sexual relationship with retired head teacher Miss Moore-Martin, and subjected her to the same psychological manipulation.
He was found not guilty of her attempted murder.
The court heard Field had drawn up a list of 100 future “targets”, including his own parents and grandparents.
Investigating senior police officer, Mark Glover, said the murderer fitted the profile of a psychopath.
“Cruel, calculating, manipulative, deceitful. I don’t think evil is too strong a word for him," he said.
“Everything is about Ben Field and Ben Field’s gain.”
Field's father, the Rev Ian Field, stepped down as Olney Baptist minister in April, shortly before his son's trial was due to start.