Third-wave feminists are the best friends a rapist could ask for. With their promotion of the idea of "rape culture"—the notion that images and culture propel men to hate and harm women—they have done more than anyone to diminish rapists' responsibility for their foul crimes. And the evidence suggests rapists are really grateful.

A recent horrific murder in Britain confirms that, 30 years on from Andrea Dworkin and Catharine MacKinnon's scaremongering over porn and its impact on the putty-like male mind, feminists are still depicting porn as the instigator of criminal activity. And in the process they're excuse-making for criminals.

Last week, Nathan Matthews, 28, was found guilty of murdering his 16-year-old stepsister, Becky Watts, during a sexually motivated kidnap plot. Matthews and his girlfriend, Shauna Hoare, had been text-messaging each other about kidnap fantasies. They also watched a lot of teen-themed and violent porn. Matthews later suffocated Becky and then sawed up her body.

It was an unspeakable crime. The court, in Bristol, found Matthews guilty both of having a criminal mind and of carrying out a criminal act: he was punished as a murderer. But in the days after his trial, something perverse happened: feminists, those who should be most keen to see the likes of Matthews held fully responsible for their foul plotting and behaviour, offered up an explanation for his wickedness that will have given him some comfort.

The revelation that Matthews was obsessed with porn became a key part of the media coverage. He was branded a "paranoid pervert." Some reporters gave the impression that the porn he watched drove him to kill. He was a "porn-obsessed recluse" who "let sick fantasies overtake him." Another said Matthews may have suffered the "desensitising effect" that apparently comes from living in a porn-saturated world: porn dulls our feelings, apparently making us cavalier about hurting others.

The hacks who focused on Matthews' obsession with porn no doubt thought they were exposing what a despicable character he is. But their reporting had a different, unwitting impact: it absolved Matthews of full culpability through claiming he had been "taken over" by porn. Some of the coverage verged on presenting Matthews as a victim; a victim of "porn culture."

Feminist commentators got closest to diminishing his responsibility. In the New Statesman, Sarah Ditum argued that Matthews, like other men who watch porn and then kill or rape, will have "learned" from all his porn what "women are supposed to be for": "something to fuck, something to use, something to hurt if you'd like to."

Ditum went so far as to suggest that many men are almost possessed by porn. "This is how porn operates," she said: "first through the eyes, and then in the mind, and then back through the body, against other bodies." So porn takes hold of us, took hold of Matthews, and drives us, automaton-like, to evil. If Matthews has read Ditum's words, he will have found comfort in them: perhaps I'm not evil, he'll have thought; perhaps I am just the slavish enactor of cultural trends. "Humans are creatures of culture," said Ditum. How surreal that it was a feminist who unwittingly absolved Matthews of wickedness through claiming that porn acted through him—through the eyes, the mind, the body—rather than this being a case of him acting upon his own warped volition.

In the Telegraph, Joan Smith argued that the "normalisation" of watching hardcore porn has made murders like Matthews' more likely. It is time, she says, to make the anti-porn message part of every child's schooling, so that they know that watching twisted porn is "not healthy or acceptable behaviour." "However liberal we might want to be," it is time to address how "desensitising" porn culture can be, she said.

Here, again, Matthews is effectively excused. He was desensitised, programmed, and if only educators had protected him from porn, perhaps Becky would still be alive. The porn made him do it.

These feminists' unwittingly friendly overtures to rapists and killers show how dangerous the idea of "rape culture" is. "Rape culture" is the name given to a vast array of mostly harmless cultural practices—from saucy magazines to sexist banter on campus—which feminists claim contribute to a social disregard and even disdain for women's equality and security. On both sides of the Atlantic, the rallying cry of third-wave feminists is that culture makes men wicked and reduces women to victims.

There are two big problems with the idea of "rape culture." The first is that it is built on some very shoddy statistics. As Christina Hoff Sommers, Cathy Young, and others have amply demonstrated, it simply isn't true that one in four women are sexually assaulted or that women in the 21st century live in a "sea of misogyny."

The second problem is that the fetishisation of culture as the cause of violence and shaper of attitudes smashes the idea of free will and moral autonomy. And this is a boon to those who have chosen, freely, to do something awful with their moral autonomy. Like rapists.

The notion that porn directly acts on society, as if it were some sentient force moving "through the eyes" and "against other bodies," is not new. It was promoted by Dworkin and MacKinnon in the 1980s and 1990s, and it was challenged, brilliantly, by feminist libertarians like Nadine Strossen, author of Defending Pornography (1995).

As Strossen argued, "a causal connection between exposure to pornography and the commission of sexual violence has never been established." And it still hasn't been: not one serious study shows any link between the availability of porn and spikes in misogynist violence. Indeed, Strossen pointed out that in countries where possession of porn is severely punished—like Saudi Arabia—women are far more likely to face everyday prejudice and violence. In the West, meanwhile, the explosion in online porn has coincided with a general fall in violence.

But one group of people have fully welcomed censorious feminists' fact-lite association of porn with rape and murder: rapists and murderers.

Strossen pointed out that in the 1980s and 90s, some men who had committed foul deeds fell back on the Dworkinite idea that the culture made them do it in an attempt to shrink their guilt. Marcia Pally, academic and feminist against censorship, wrote about how in the mid-1980s, when the court refused to declare him insane, Ted Bundy started "collecting information attesting to the negative effects of pornography," in order to show that wicked images made him wicked. He started quoting academic research as part of his attempt to "bolster his pornography-made-me-do-it claim."

At the time of Bundy's execution in 1989, Dr Gene Abel, Professor of Psychiatry at Emory University, wrote of Bundy's "false beliefs" that were designed to "explain his behaviour." He summed up Bundy's excuse-making as: "It wasn't my fault, these are pornographic things that I've seen." In the Harvard Civil Rights—Civil Liberties Law Review, Barry Lynn wrote of how in the 80s and 90s, "some defendants [were using] pornography as a convenient excuse for their actions."

How long until Nathan Matthews uses the porn defense, too? The case is already being made for him—by feminists. What a damaging idea "rape culture" is, pushing for censorship, spreading fear about sex, and, worst of all, allowing Bundy, Matthews and other nasty men to present themselves as victims, effectively. Rapists could not ask for a more favourable ideology than rape culture. It's what they've been waiting for. The feminist-rapist alliance—it has come to this.