Lord Lucan's son has admitted he hoped the missing British aristocrat was involved in the death of family nanny Sandra Rivett.
But in his first in-depth interview about the 1974 murder, George Bingham insisted he was certain his father was not the killer.
Rivett, 29, was found dead at the home of the peer's estranged wife in Belgravia, London. She had been bludgeoned with a lead pipe.
The nanny's attacker turned on Lady Lucan, beating her severely before she managed to escape and raise the alarm at a nearby pub.
Lucan's car was later found abandoned and soaked in blood in Newhaven, East Sussex, and an inquest jury declared the wealthy aristocrat was the killer a year later.
What happened to Lucan remains a mystery and he was officially declared dead by the High Court in 1999.
Bingham, 45, who was in the house with his siblings at the time of the attack, said it was "extraordinarily unlikely" that his father was the killer or paid somebody else to carry out the atrocity.
"People will question my judgment. Others will dispute it," he told the Daily Mirror.
"But what I am certain of is dad was not the prime mover in the situation.
"Weirdly, however, I do hope he was partly culpable because it makes me feel better."
The former merchant banker added: "I would rather that than have my father leaving us for no apparent reason."
"I've always thought it extraordinarily unlikely my father went into our family home, wandered down and killed anybody with a piece of lead piping for the love of his children, while those very children might well have come downstairs and witnessed this appalling carnage.
"Hiring a stranger, a killer no less, has to be even less likely."
Following his disappearance, there were reported sightings of Lucan in Australia, Ireland and South Africa.
His brother, Hugh Bingham, said he was confident Lucan escaped to a new life in Africa.