It would be foolish to think that the execution of Troy Davis in Georgia on Wednesday local time would be enough to prompt a change of heart in the US legal system regarding the death penalty.

The tide of public opinion is slowly turning against state-sanctioned killing - independent research shows a decline from 80 to 62 per cent support since the early 1990s - but lawmakers and judges show no sign of moving.

The US Supreme Court, which devoted 23 words - less than a 140-character tweet, as the New York Times remarked - to rejecting Davis' bid for a stay of execution, has a conservative majority and has generally supported the death penalty since it reinstated it in 1976.

The Davis case should excite particular outrage against the ultimate penalty because of the doubts that swirled around his conviction for the 1989 killing of an off-duty police officer in Savannah. Not a shred of physical evidence connected Davis to the crime and seven of nine eyewitnesses against him have recanted their testimony

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High-profile figures came to his support, including the Pope and former President Jimmy Carter, the latter saying that the doubt surrounding Davis' conviction called the entire death-penalty system into question. But the death penalty is repugnant even without the uncertainty that plainly attends on Davis' guilt.

The US is the only western nation still using the death penalty: keeping it company are such bastions of freedom as China, Iran, North Korea and repressive regimes in Africa and the Middle East.

When the US Supreme Court effectively banned the use of capital punishment in 1972, it found that it was unconstitutional because it was "cruel and unusual punishment", since it was "wantonly and freakishly" imposed - a coded way of saying that it was disproportionately carried out on African-Americans.

But the words are useful in any case: there can be no crueller and more unusual punishment than death. The proposition, rooted in our basest and most primitive urges, that we should express our respect for the sanctity of human life by taking it, would be comic if it were no so grim in practice.

On Wednesday, Troy Davis became the 1269th person executed in the US since the death penalty was reintroduced. He joins countless thousands of others. How many more must die before the self-proclaimed leader of the free world turns its back on this barbarism?