A mother has revealed how she forgave her son for murdering her four-year-old daughter when he was 13 and why she continues to visit him in prison a decade after the attack which he launched in revenge against her.

Charity Lee was at her waitressing job in Abilene, Texas, on February 5, 2007, when police arrived to tell her her four-year-old daughter Ella was dead.

The child had been stabbed 17 times by her brother, Paris, who was 13 at the time.

Paris picked up the knife and attacked his half-sister as she slept in her bed after convincing their babysitter to go home. He then called 911 and admitted to the killing.

He later told his mother that he carried out the murder to 'destroy' her in revenge.

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Paris Lee Bennett murdered his four-year-old sister Ella when she was four and he was 13 in 2007. They are pictured years before the attack

He was angry, he said, that she had briefly relapsed into drug use years earlier after getting herself clean.

Charity overcame heroin addiction not long before she became pregnant with Paris. When she relapsed, it was with cocaine.

Paris is 24 now and remains behind bars in Texas where his mother, who has since had another baby, visits him.

He was sentenced to 40 years imprisonment, the maximum for juvenile murder, in 2007.

Their story is told in The Family I Had, a documentary which will be aired on Investigation Discovery on Thursday December 21 at 9pm EST.

The documentary follows Lee, 44, from Savannah, Georgia, where she now lives, to Texas to visit Paris behind bars.

'I have forgiven Paris for what he did but it's an ongoing process.

'If he was free, I would be frightened of him,' Lee told The New York Post in an interview to promote the documentary.

Charity visits Paris in prison regularly and is determined to help him by loving him and trying to get him psychological help. He was diagnosed as a sociopath in 2009. The pair are pictured after his prison high school graduation in jail

Paris remains in jail. He is 24 years old now and maintains that he is not mentally ill despite being diagnosed as a sociopath

Paris in his prison high school graduation photograph

On the night of February 5, 2007, Paris, who has an IQ of 141, convinced his and Ella's babysitter to go home.

Anything above 140 is considered genius.

He then went into his sister's bedroom and attacked her.

Afterwards, he called a school friend on the phone then he phoned 911.

His harrowing phone call to emergency workers is included in the trailer for the documentary.

'I accidentally killed somebody,' he wailed.

The operator responded: 'You think you killed somebody?'

Paris, through sobs, cried back: 'No I KNOW I did.

'My sister...I feel so messed up.'

He was taken into custody before authorities alerted Charity to what had happened.

Charity with Ella shortly before her death. Ella was asleep and her mother was at work when she was killed

Paris with Ella when they were younger. His mother said he 'fell in love' with her when she was born

'An easy target': Ella, four, was asleep in her bed when her brother attacked her. Until that night, there had been no signs he would hurt her and the pair played happily

He told her first that he had suffered a hallucination but then later claimed he was in his right mind when he stabbed Ella and said it was for revenge against their mother.

After abusing drugs before either of the children was received, she relapsed briefly when Paris was nine and Ella was two.

The fact is, it made him angry and he chose to handle it that way [by killing Ella],' Charity said this week.

Despite being diagnosed as a sociopath, Paris maintains that he has no health issues.

In a jailhouse interview for the documentary, he told filmmakers: 'I chose to do my crime and I take full responsibility for my crime.

'And I wouldn’t say there was a predisposition to what happened.

Devoted: Charity with Paris when he was born. She overcame a heroin addiction shortly before he was conceived and relapsed briefly with cocaine when he was nine - something he did not forgive her for

Paris and Charity not long before the attack when he was 13. He denies that he suffers mental illness

Paris told his mother in 2007 that he killed Ella to 'destroy' his mother's life in pay back for her brief relapse into drug use years earlier. He is seen above, left, before the murder and right, aged in his 2007 mugshot

'I’m not insane and I don’t suffer from any mental illness.'

Four years ago, Charity had another baby, a boy who she named Phoenix.

Paris has never met him.

Because of he murdered a child, he is not allowed to have any contact with children including prison visits.

His mother believes the separation has allowed her to continue to visit him.

'If Paris wasn’t in prison or was able to meet Phoenix, I would have to do a lot more soul-searching,' she said.

She said she will never 'abandon' her son despite his 'dark' ways.

Charity had another baby, Phoenix, four years ago . He has not been to visit Paris in jail

Paris's mother described him as 'manipulative' and bright. He has written letters to Phoenix, who is now the same age Ella was when he murdered her. He has never met his younger brother

'I sometimes have to say to myself [during visits]: "Okay, Charity, take a breath, you know how Paris is wired,"'

'But I am not going to be that parent who abandons their kid,' she told The Post.

Charity, 44, now travels to prisons and gives talks on how the victims of violence can respond to empower themselves

In the trailer for the documentary, she said: 'Paris knows somewhere inside of him, he is dark.'

To cope with her grief, Charity set up The Ella Foundation, a nonprofit organization which empower people affected by violence.

She was prompted to set it up in part when in 2007, a month after Paris's arrest, the Texas Youth Commission, which had been looking after him, was indicted for the physical and sexual abuse of children in its care.

Now, she visits prisons to speak about how to respond to violence in ways which can improve a person's life instead of destroy it.

The Family I Had will be shown on Investigation Discovery on Thursday December 21 at 9pm EST.