Chris Watts' prison pen pal gives disturbing theory on why he murdered wife and two daughters

A prison pen pal of Chris Watts has given a disturbing insight into why the Colorado father might have murdered his pregnant wife and two young daughters, in one of the most horrific crimes in recent memory.

The family killer is currently serving five life sentences without parole for the murders of his 15-week pregnant wife, Shanann, and their two daughters on 13 August 2018, having narrowly avoided the death penalty as part of a plea deal.

Watts was told by a judge in 2018 that his killings were the most 'inhumane and vicious crime' he had seen, after he strangled his wife to death before suffocating his two young daughters and dumping their bodies in a nearby oil storage site.

He had initially pretended that his family were missing, while he was also sleeping with another woman, Nichol Kessinger, who has since revealed the last texts that he sent her before it became clear he was behind their murder.

Watts allegedly told his cellmate about his relationship with Kessinger, even having the audacity to place the blame on his co-worker for his horrific crimes, yet he seems to have a different attitude when speaking with his prison pen pal, crime author Cherlyn Cadle.

She wanted to learn more about what led to him committing such unthinkable acts with his own family, and she suggests that it was his 'dark fantasies' that led to such a gruesome ending.

Shannan and Chris Watts with their daughters Bella and Cece (Handout)

Cadle described their secret romance as 'very sexual, very twisted, very mixed up' and suggested this was 'part of why I believe he did what he did,' according to an interview with the Daily Mail.

She believes he got some kind of sexual thrill from telling her about his sex life, as well as the heinous crimes he committed.

"I think there's some sort of fetish there," she said. "To say things to shock people. And he told me, who he saw as a mother figure, those details about different sex acts he did with a woman."

The last texts sent by his wife Shannan suggest that it wasn't just sexual fantasies that played a part, as he seemed to be unsure about becoming a father for the third time.

"Chris told me last night he's scared to death about this third baby, and he's happy with just Bella and Celeste and doesn't want another baby," she wrote in a text to her best friend, ahead of a planned gender reveal.

She continued: "He said we're not compatible anymore. He refused to hug me after he said he will try to 'work it out'."

Watts had started an affair with Kessinger before murdering his wife and kids (Handout)

"I haven't slept most of the week," read a third message. "My eyes burn from crying so much.

"I cancelled the gender reveal. Nicki [friend Nicki Atkinson] is going to tell me [the baby's gender] today. I need happy news today.

"This baby in my belly deserves his full love either way."

Watts' cellmate suggests that he has turned to religion since arriving in prison, and not that it's any real consolation, it appears that he deeply regrets his actions, and it's something that will haunt him for the rest of his life.

According to Watts’ 2019 prison confession, he admitted that he hears his daughter Bella's last words every time he closes his eyes, after she pleaded with him to avoid the same fate as her sister Celeste (CeCe) after he suffocated both his young children.

"Right now I'd have a 5-year-old ... a 3-year-old ... and more than likely, a one-month old son…and a beautiful wife…and right now it's just me," he told investigators.