On the morning of 11 January Mostafa Ahmadi Roshan, the deputy head of Iran's uranium enrichment facility at Natanz, was in his car on his way to work when he was blown up by a magnetic bomb attached to his car door. He was 32 and married with a young son. He wasn't armed, or anywhere near a battlefield.

Since 2010, three other Iranian nuclear scientists have been killed in similar circumstances, including Darioush Rezaeinejad, a 35-year-old electronics expert shot dead outside his daughter's nursery in Tehran last July. But instead of outrage or condemnation, we have been treated to expressions of undisguised glee.

"On occasion, scientists working on the nuclear programme in Iran turn up dead," bragged the Republican nomination candidate Rick Santorum in October. "I think that's a wonderful thing, candidly." On the day of Roshan's death, Israel's military spokesman, Brigadier General Yoav Mordechai, announced on Facebook: "I don't know who settled the score with the Iranian scientist, but I certainly am not shedding a tear" – a sentiment echoed by the historian Michael Burleigh in the Daily Telegraph: "I shall not shed any tears whenever one of these scientists encounters the unforgiving men on motorbikes."

These "men on motorbikes" have been described as "assassins". But assassination is just a more polite word for murder. Indeed, our politicians and their securocrats cloak the premeditated, lawless killing of scientists in Tehran, of civilians in Waziristan, of politicians in Gaza, in an array of euphemisms: not just assassinations but terminations, targeted killings, drone strikes.

Their purpose is to inure us to such state-sponsored violence against foreigners. In his acclaimed book On Killing, the retired US army officer Dave Grossman examines mechanisms that enable us not just to ignore but even cheer such killings: cultural distance ("such as racial and ethnic differences that permit the killer to dehumanise the victim"); moral distance ("the kind of intense belief in moral superiority"); and mechanical distance ("the sterile, Nintendo-game unreality of killing through a TV screen, a thermal sight, a sniper sight or some other kind of mechanical buffer that permits the killer to deny the humanity of his victim").

Thus western liberals who fall over one another to condemn the death penalty for murderers – who have, incidentally, had the benefit of lawyers, trials and appeals – as state-sponsored murder fall quiet as their states kill, with impunity, nuclear scientists, terror suspects and alleged militants in faraway lands. Yet a "targeted killing", human-rights lawyer and anti-drone activist Clive Stafford Smith tells me, "is just the death penalty without due process".

Cognitive dissonance abounds. To torture a terror suspect, for example, is always morally wrong; to kill him, video game style, with a missile fired from a remote-controlled drone, is morally justified. Crippled by fear and insecurity, we have sleepwalked into a situation where governments have arrogated to themselves the right to murder their enemies abroad.

Nor are we only talking about foreigners here. Take Anwar al-Awlaki, an Islamist preacher, al-Qaida supporter – and US citizen. On 30 September 2011, a CIA drone killed Awlaki and another US citizen, Samir Khan. Two weeks later, another CIA-led drone attack killed Awlaki's 21-year-old son, Abdul-Rahman. Neither father nor son were ever indicted, let alone tried or convicted, for committing a crime. Both US citizens were assassinated by the US government in violation of the Fifth Amendment ("No person shall be deprived of life without due process of law").

An investigation by Reuters last October noted how, under the Obama administration, US citizens accused of involvement in terrorism can now be "placed on a kill or capture list by a secretive panel of senior government officials, which then informs the president of its decisions … There is no public record of the operations or decisions of the panel … Neither is there any law establishing its existence or setting out the rules by which it is supposed to operate."

Should "secret panels" and "kill lists" be tolerated in a liberal democracy, governed by the rule of law? Did the founders of the United States intend for its president to be judge, jury and executioner? Whatever happened to checks and balances? Or due process?

Imagine the response of our politicians and pundits to a campaign of assassinations against western scientists conducted by, say, Iran or North Korea. When it comes to state-sponsored killings, the double standard is brazen. "Actions are held to be good or bad, not on their own merits, but according to who does them," George Orwell observed, "and there is almost no kind of outrage … which does not change its moral colour when it is committed by 'our' side".

But how many more of our values will we shred in the name of security? Once we have allowed our governments to order the killing of fellow citizens, fellow human beings, in secret, without oversight or accountability, what other powers will we dare deny them?

This isn't complicated; there are no shades of grey here. Do we disapprove of car bombings and drive-by shootings, or not? Do we consistently condemn state-sponsored, extrajudicial killings as acts of pure terror, no matter where in the world, or on whose orders, they occur? Or do we shrug our shoulders, turn a blind eye and continue our descent into lawless barbarism?

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