On 23 September, 40-year-old Teresa Lewis will become the first woman to be executed in the state of Virginia for almost a century. She'll also be the first woman put to death in the US since 2005. Considering that, in the intervening five years, around 220 men will have been executed, it puts it into perspective: executing women is unusual. Of more than 1,200 executions carried out since the US supreme court reinstated capital punishment in 1976, only 11 were of women. And each time that happens, it's stunningly bad PR for an increasingly unpopular facet of the American justice system.

The facts of the Lewis case are fairly gruesome. In 2002, she was convicted of persuading two men to kill her husband and stepson to collect a $250,000 life insurance policy. In return, she promised them a portion of the money, and sex with her and her 16-year-old daughter.

Lewis pleaded guilty. So you might think that this is a cut-and-dried death penalty case. But I don't think so, and if you look at the facts, and consider the way the death penalty is administered in the US generally, it leaves more than a bad taste.

A forensic psychiatrist testified that Lewis has an IQ of 72, placing her in the "borderline range of intellectual functioning". Her co-accused, Rodney Fuller and Matthew Shallenberger – the two gunmen who actually did the killing – were sentenced to life imprisonment (Shallenberger actually committed suicide a few years later). And although the judge acknowledged that Lewis had led police to the men, he described what she had done as "horrible and inhumane", and determining she had masterminded the whole thing, sentenced her to death.

At appeal, her new lawyers argued her trial attorneys should have presented hundreds of pages of medical records that showed her dependency on prescription drugs and that she was too easily led by other people to have plotted the murders. A psychiatrist specialising in addiction testified that Lewis's mental state before, during and immediately after the killings was "significantly impaired" as a result of developmental disabilities, borderline intellectual function and that dependence on drugs.

The defence then produced vital evidence – a letter from Shallenberger admitting it was he, not Lewis, who planned the murder. "The only reason I had sex with the mother was," he wrote, "to get her to fall in love with me so she would give me the insurance money."

But the appeal court upheld the sentence.

In the eight years Lewis has been on death row, she is said to have been a model prisoner. But on death row, good behaviour counts for nothing. In a month's time, she will become the first woman executed in Virginia since 1912.

Executing men has become routine – most of the time, the deaths warrant only a small mention in the local newspaper. But Lewis's execution will, it is to be hoped, once again draw worldwide attention to the fact that the United States is on the roll call of countries with the less-than-salubrious distinction of carrying out the highest number of executions, along with China, Iran, Saudi Arabia, Pakistan, Congo, Egypt and Iraq – states with which the US perhaps ought to feel a little uneasy being compared.

And later this year, there's a good chance that a British passport holder, 51-year-old Linda Carty, will join Lewis. I have written about Carty before: her trial was seriously flawed, and if, like Lewis, she is also given a 2010 execution date, it will draw even more attention to the US's dire record on capital punishment.

Richard Dieter, executive director of the Death Penalty Information Centre, told me that because of juries' reluctance to dish out death sentences these days, it's fair to assume that if Teresa Lewis was in court today, she probably wouldn't be sentenced to die. "Particularly in Virginia," he said. "There was just one death sentence in that state last year, and to say Lewis is the worst of the worst is a stretch. Given the mitigating evidence and the fact the shooters got life sentences, it strikes you as unfair in the way it played out."

There's not much chance that Virginia governor Bob McDonnell will commute Lewis's sentence to life. Considering McDonnell is pro-life, opposes same-sex marriage and holds an A-rating from the National Rifle Association for his gun rights advocacy, you can guess where he stands on the death penalty.

In the past few years, serial killer Andre Crawford was spared the death penalty for the murders and rapes of 11 women on Chicago's south side – he got life in prison. In Virginia, Haiyang Zhu, a former Virginia Tech student who murdered and beheaded one of his fellow students, got life in prison.

If you agree with the death penalty, you must also agree that the ultimate punishment should be meted out fairly. And the simple fact is, it isn't.