The Colorado man convicted of slaughtering his family says he never would have killed his pregnant wife and two daughters if he hadn’t met his co-worker mistress.

“If I had not met Nikki, I would never have killed my family,” Chris Watts, 34, confessed in a prison letter — revealing that he had been seeing lover Nichol Kessinger for two months when he decided to murder his wife Shanann and daughters Bella, 4, and Celeste, 3, according to the Daily Mail.

He wrote the letter to prison pen pal Cherlyn Cadle, forming the basis of her new book “Letters from Christopher.”

Watts, who is serving multiple life sentences for the August 2018 murders, told Cadle that he thought Kessinger was his “soulmate” and felt no remorse after killing his family.

“All I could feel was now I was free to be with Nikki,” he wrote after the slaughter. “Feelings of my love for her was overcoming me. I felt no remorse.”

Watts also revealed how he slipped Shanann the potent painkiller Oxycodone in the hopes she would miscarry — making it easier for him to be with Kessinger, who reportedly told Watts she wanted to “give him his first son.”

Watts later described in chilling detail how he smothered his daughters before getting back into bed and strangling his wife.

He said he attacked his daughters twice when his first attempts to suffocate them with a pillow were unsuccessful.

“I went to the girls’ room first, before Shanann and I had our argument,” he wrote to Cadle.

“I went to Bella’s room, then Cece’s room and used a pillow from their bed,” the monster wrote.

He then went back to bed and continued arguing with his wife before strangling her.

“After Shanann had passed, Bella and Cece woke back up. I’m not sure how they woke back up, but they did,” he wrote in the April 2019 letter.

“Bella’s eyes were bruised and both girls looked like they had been through trauma.”

Watts previously confessed to driving his daughters Celeste, 3, and Bella, 4, to a Weld County Oil site where he dumped his wife’s body and then smothered the girls for a second time.