The story of Jamal Khashoggi, a onetime Saudi insider turned outspoken critic who disappeared in the Saudi embassy in Istanbul, is now one well told.

The evidence, according to the UN, points to a brutal and premeditated killing, perpetrated by Saudi Arabian officials as a warning to all dissenters.

Like countless others through history — including Julius Caesar, Abraham Lincoln, Martin Luther King and the Kennedy brothers — Khashoggi was assassinated.

In recent years assassins have also claimed the lives of a Russian defector, the brother of the North Korean leader, and a Maltese investigative journalist.

As historian and author Michael Newton puts it: "One thing which is distressing to the historian of assassinations is the history is never finished."

But as history has moved from Caesar to Khashoggi, the seeming constant of assassination has evolved.

More personal

Motivation is an important factor in any crime, and the calculated murder of a public figure is no exception.

But the driving factors for assassins are shifting.

According to Dr Newton, 19th century assassinations were often motivated by idealism, usually with a coherent political ideology attached.

"Even if you look at John Wilkes Booth's assassination of Abraham Lincoln, obviously he was performing a wicked deed, but by his own lights he was in the right," he says.

America's civil war president Abraham Lincoln was assassinated in 1865. ( Wikipedia Commons: Currier & Ives )

But over time the politics of assassinations have changed, with targeted slayings becoming increasingly senseless, and detached from principles.

The 20th century saw the rise of the pathologically narcissistic assassin, primarily driven by a desire to gain fame and notoriety.

"When you get to the attempted assassination of Ronald Reagan, the meaning of the event is really absent, it's just about fame," Dr Newton says.

The would-be assassin of Ronald Reagan, John Hinckley, was said to be motivated by fame. ( Getty: Battmann )

Research suggests that in the past 20 years, however, assassins have tended to be motivated by personal grievances, no matter how senseless.

This was the discovery of Dr Reid Meloy, a forensic psychologist who has been a consultant to the FBI for almost two decades.

"It's a retaliatory grievance-filled act of violence where they feel they are completely justified in what they do," he explains.

Less famous

In the aftermath of the high-profile US assassinations of the 1960s and 70s, presidents and other prominent figures have made themselves more difficult targets.

"It's not accidental that the political assassinations that have happened in the last 10 years have been low-level events," Dr Newton says.

British MP Jo Cox, who was killed outside a local library in her electorate, is now a more typical case.

Jo Cox was at a local library when she was shot by a man with links to far right extremist groups. ( Getty: Dan Kitwood )

"Those people are much closer to the public than prime ministers and presidents are," Dr Newton says.

"But if you go back it was easy to come close to, in American terms, the president."

This was certainly true of the 60s, a decade which saw the assassinations of civil rights activist Martin Luther King, John F Kennedy, and later, his brother Robert.

"There's lots of footage of Bobby Kennedy in the 1968 election campaign at the centre of the melee of people trying to shake his hand or to touch him," Dr Newton says.

Robert Kennedy was regularly surrounded by crowd's during his 1968 presidential campaign. ( Wikipedia Commons: United States Library of Congress )

"It's not at all surprising that Sirhan Sirhan could get close to Bobby Kennedy because hundreds of thousands of people got close to him in that time.

"Now such a thing just doesn't happen."

More gangster

Historically, political leaders were the common targets of assassination, but in the 21st century, dissidents and journalists have become frequent victims.

"The two major forms of assassination are those performed by the powerless and those performed by the powerful," Dr Newton explains.

"When the powerless attack a president or a monarch, they are trying to assert a little bit of power in a situation where they themselves feel they have none.

"But assassinations from above, when they are performed by states, as is the case with Russia and with Saudi Arabia, is precisely the opposite. It's assertion of the state's power and its reach."

Other high-profile assassinations of the 21st century include Maltese investigative journalist Daphne Caruana Galizia and Russian defector Alexander Litvinenko.

In each case, the state is widely suspected of ordering the killing.

Maltese journalist Daphne Caruana Galizia was killed by a car bomb close to her home. ( Getty: Mathew Mirabelli )

"There's a gangster element to such killings in that you're showing that no-one is exempt, no-one is safe from you," Dr Newton says.

"It's disturbing … and these recent killings have led people to feel, particularly in the political climate as it exists now, that rules are breaking down.

"These acts, those deeds of violence, they have their propaganda effect, and it's the propaganda effect of intimidation and fear."

But no more effective

Many assassins hope to change the course of history in some way, and it easy to think that their actions could shape an era.

However, according to economist Ben Olken, who investigated the effect of almost 300 assassination attempts on political leaders around the world, it's not so straightforward.

"We tried to find all of the assassination attempts on leaders, either presidents [or] prime ministers, covering the period from 1875 all the way to 2004," Professor Olken says.

He found 24 per cent of the attempts were successful.

"In others the bomb goes off, the gun is fired, but the target survives," he says.

On average, Professor Olken found that an autocratic country was only 13 per cent more likely to become a democracy following the assassination of an autocratic ruler.

Assassinations that take place in a democracy, he says, rarely have a destabilising effect.

"A successful assassination attempt of a democratic leader does not lead that country, on average, to fall back and become a dictatorship relative to a failed attempt," he says.

Dr Newton says assassins mistakenly conflate institutions with the public figure at their helm.

"When you kill the president, there's a feeling that you are killing the presidency," he says.

"[But] what naturally happens and rightly happens is that's not the case; you kill the president and then there's the vice-president."

The assassination of Martin Luther King Jr did not bring an end to the US civil rights movement. ( Wikipedia Commons: Minnesota Historical Society )

Despite the shocking murders of the Kennedys, King and other political figures in the 1960s, life in the US went on.

"The presidency was unharmed, democratic institutions resolved themselves around these events and American life on one level was completely untouched by them," Dr Newton says.

But then again, perhaps the image of Kennedy driving through Dallas in an open convertible, or Khashoggi entering the golden doors of the Saudi embassy, is a legacy in itself.