A female ISIS volunteer who later fled the fanatical terrorists described a harrowing life of beheadings, sexual violence and torture that awaits unsuspecting women who join the militants.

“The worst thing I saw was a man getting his head hacked off in front of me. I said, ‘Enough.’ I decided no, I have to leave,” said the woman, identified as Khadija, a former teacher who joined the brutal Khansaa Brigade.

The unit was assigned to patrol the streets of Raqqa, Syria, and ensure that women wore clothes that concealed their faces and slavishly adhered to ISIS’s brutal Sharia law.

Those who broke the rules were badly beaten and lashed by Umm Hanza — a terrifying, physically intimidating leader Khadija described as “not a normal female” who had a sadistic love of violence.

“She’s huge, she has an AK [47 machine gun], a pistol, a whip, a dagger, and she wears the niqab,” the disaffected volunteer told CNN.

Hanza, she said, warned her, “We are harsh with the infidels, but merciful among ourselves.”

Even worse than the brigade’s treatment of women, she said, was the sexual violence ISIS thugs — especially the foreign fighters — committed against their “wives” after forced marriages.

Khadija, 25, said she grew up in Syria and started out as an activist against the regime of President Bashar al-Assad.

“Everything around us was chaos,” she said. “Free Syrian Army, the regime, barrel bombs, strikes, the wounded, clinics, blood — you want to tear yourself away, to find something to run to. My problem was I ran away to something uglier.”

A Tunisian man she met online lured her into joining ISIS with phony promises that it wasn’t a terrorist group and that they would get married.

“He would say, ‘We are going to properly implement Islam. Right now we are in a state of war, a phase where we need to control the country, so we have to be harsh.’ ”

And at first, Khadija embraced her new role as an ISIS enforcer.

“At the start, I was happy I was carrying a gun,” she told the network. “It was something new. I had authority. I didn’t think I was frightening people.”

But the casual, everyday brutality led her to question her own judgment.

Finally, a particularly violent crucifixion of a 16-year-old accused of rape convinced her she needed to get out.

She slipped over the border to Turkey just days before US-led allied airstrikes.

Now, she only wants to be the person she was before being ensnared in the terrorists’ clutches.

“A girl who is merry, who loves life and laughter . . . who loves to travel, to draw, to walk in the street with her headphones listening to music without caring what anyone thinks,” she said.

“I want to be like that again.”