On a November day, a much-loved but also much-mistrusted leader appears in public to boost his political fortunes. Many voters adore him as a bold reformer who aims to silence the drums of war. Others detest him as a wrecker and a renegade. In full view of the crowds, shots from a solitary sniper leave the politician mortally wounded. He dies soon afterwards. Instantly, both his past record and future plans become a matter of unending dispute. The most outlandish theories swarm around the head of the murderer, a lonely malcontent. None is ever proved; none is ever quashed. Friends and enemies alike believe that the gunman has altered the course of history for good.

Dealey Plaza, Dallas, on 22 November 1963? No: Kings of Israel Square, Tel Aviv, on 4 November 1995. The killing of Israeli prime minister Yitzhak Rabin by the religious extremist Yigal Amir, and its bitter aftermath, shows how deeply the assassin’s tale has bedded down into the mental soil of modern life. True, Rabin – who had agreed the Oslo accords with Yasser Arafat, recognised the Palestine Liberation Organisation and signed a pact with Jordan – left more solid proofs of his vision for peace than John Fitzgerald Kennedy. For every historian who thinks that JFK would have scaled back the US military commitment in Vietnam, another suspects that he would have let it escalate. That’s another common feature of the assassination narrative we tell so fluently now. Along with all the conspiracy theories come the fretful counter-factuals, a clinging bog of what-ifs and might-have-beens.

However much, or little, the assassin’s blade, bomb or bullet may reroute the march of history, it has beyond all doubt reset our political and even moral compass. We dread, despise and sometimes even pity the assassin. As those shades-clad minders whisper into their lapels, we see that whole branches of the security apparatus make it their business to thwart him. It’s still, in general, him, although the Russian revolutionary Vera Zasulich triggered imitators when she nearly killed a repressive governor of St Petersburg in 1878; amazingly, she was acquitted. Yet the underlying assumption behind state policy in many countries remains this: assassination works.

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These days, it requires no sociopathic crack shot skulking amid the chimney pots. On 1 November, yet another unmanned US drone strike killed the latest leader of the Pakistani Taliban, Hakimullah Mehsud, in north Waziristan. That execution was part of a long-term strategy of decapitation of the Taliban and al-Qa’ida from the skies. Britain does not itself operate armed drones over Pakistan, although legal wrangles surround its supply of “locational intelligence” to US forces. In Afghanistan, however, the RAF’s Reaper drones have since 2007 deployed about 350 weapons. Official secrecy means that data about the targets of these missions remains vanishingly scarce. For anti-drone campaigners such as the human rights charity Reprieve, the picture is clear enough. To them, the UK has become, if not an assassin state, then the intimate accomplice of one. As for the US itself, on 2 May 2011 in Abbottabad, Pakistan, Operation Neptune Spear won near-universal public acclaim for a policy of “targeted killing” when it located and then terminated Osama Bin Laden.

So the ultimate tool in “asymmetric warfare”, wielded for so long from below, now comes – in every sense – from above. Indeed, assassination began as a force in Western history not with the brooding of the powerless but with the feuding of the powerful. The era of selective top-level execution dawns not with John Wilkes Booth, firing his Derringer at Abraham Lincoln in Ford’s Theatre, Washington, on 14 April 1865, but 1,900 years earlier: on the Ides of March 44BC. When Brutus, Cassius and the other “Liberators” stabbed the “perpetual dictator” Julius Caesar as he arrived at the Senate in Rome, they inaugurated a dozen years of fratricidal bloodletting. It ended in an outcome exactly contrary to the one they desired. The conspiring aristocrats did not restore the republic. They ushered in an empire.

Assassins do change history – but often not at all as they wished or dreamed. At Ford’s Theatre, Booth did not free the beaten (white) South from submission to the Union at the close of the American Civil War. Rather, he ensured a Confederate humiliation. You might have imagined that Booth, an actor, would have known that. He had played in Shakespeare’s Julius Caesar alongside his brothers. And he acted the part of Mark Antony, who correctly foresees that “Caesar’s spirit, raging for revenge … Shall in these confines with a monarch’s voice/ Cry ‘havoc’ and let slip the dogs of war”.

JFK and Lee Harvey Oswald aside, the other assassin’s anniversary to make waves now involves the most dreadful sequence of unintended consequences in history. When, on 28 June 1914, in Sarajevo, Gavrilo Princip fired into the open car of Archduke Franz Ferdinand and his wife Sophie, the Bosnian Serb nationalist yearned for his subject nation’s liberation from the Austro-Hungarian yoke. Instead, the First World War broke out within six weeks. Havoc, indeed. By 1918, when the tide of blood finally ebbed, Serbian ambitions would be swallowed up in the unstable new state of Yugoslavia. The “collateral damage” here, always a risk for the would-be assassin (Princip unwittingly murdered Sophie as well), arguably spread out to claim more than 15 million souls.

Yet, from the upper-crust idealism of Brutus and Cassius onwards, assassination has often had a remarkably good rep. After the Renaissance, the philosophical justification of “tyrannicide” could defend more or less any selective slaughter of a hated ruler. And Scotland, by the way, took the lead in death by firearm, without the messy close encounter the traditional sword or dagger required. Some history books maintain that the pistol shooting in 1584 of William the Silent, who led the Dutch revolt against Spanish rule, was the (literal) starting gun for gunpowder-fuelled assassination. In 1570, though, a disgruntled member of the outlawed Hamilton clan killed the Earl of Moray, Scotland’s “Good Regent”, with a yard-long carbine at Linlithgow. So the Scots fired first.

Until the 20th century, one can survey the apologists for assassination and find in their reasons and motives little more than a high-falutin’ gloss on sectarian hatred or dynastic rivalry. Michael Newton’s Age of Assassins posits “the decline of the Western assassin” from misplaced idealism to celebrity-fixated narcissism (as in Mark Chapman, the killer of John Lennon). The argument does appeal. Look back beyond the time of John Wilkes Booth, though, and it’s hard to spot many noble principles at work.

With the age of genocidal despots, the moral ambiguities thicken. Would you have killed Adolf Hitler prior to, or even during, the Second World War? The finest fantasy of precautionary decapitation never even names Hitler. Geoffrey Household’s brilliant thriller Rogue Male, published in 1939, depicts a freelance English hunter who has tried to bag “the biggest game on earth”. He is a righteous avenger “consecrated by his own anger to do justice where no other hand could reach”. But still he fails.

Anti-Nazis in Germany pondered the Führer-slaying question at length. In November 1939, an affable Swabian carpenter named Johann Georg Elser travelled to Munich with the home-made bomb he had crafted for months. Elser was an ordinary bloke who loathed the Nazis. He hid his time-bomb in the beer hall where Hitler was due to speak. The Führer left a few minutes before it detonated. Eight people died. The Nazi hierarchy – with a mindset that foreshadowed later investigations – could not believe that Elser had acted alone. He had. Yet even this simple, just man inflicted “collateral damage”.

As for the much better-known Von Stauffenberg bomb plot of 20 July 1944: what if that oak table leg in a hut at the Wolf’s Lair had proved less sturdy? Here, as so often, the swamp of counter-factual speculation drags the mind down into murk. Yes, Hitler’s death might have accelerated an Allied truce with Germany. But that armistice might have given surviving elements of the Nazi regime free rein to do their worst for years within the diminished borders of their country.

So the assassin wears a double moral face today. On one hand, the sinister loner – with or without shadowy puppet masters – steadies the crosshairs at the upper window, ready to take down a Kennedy, a Martin Luther King or a Rabin, and so break a people’s heart. On the other, the righteous predator – entirely inhuman, when in drone or missile form – snatches one life in an act of hygienic pre-emption so as to redeem countless others. Dror Moreh’s extraordinary documentary, The Gatekeepers, interviews six former chiefs of the Israeli security service, Shin Bet. You will never find a deeper, or more troubling, symposium on the ethics of “targeted killing”. Avi Dichter, Shin Bet head from 2000 to 2005, remembers with professional pride the “clean and elegant” explosive mobile phone that did for Hamas bomb-maker Yahya Ayyash.

But as the history of assassination – or the state of Gaza – amply prove, in the long run there are no clean shots. Killers in the cause of security often come to grief. Killers in the service of strife and hatred can expect a more appropriate result. Carmi Gillon led Shin Bet at the time of Rabin’s death. He has no doubt Amir, who slipped through his net, dealt a mortal blow to peace: “He succeeded big time.”