Sick letter writer Beattie believes he is being targeted by British secret service agents
Beattie served 16 years in jail for killing ambulance controller and father-of-two Denis Mullen five decades ago.
Last week he was found guilty in Dungannon Magistrates Court of sending Denise a threatening letter - purporting to come from the Mid-Ulster UVF - in an attempt to coerce her into dropping legal proceeding against him.
In an interview with the Sunday World, Beattie said he was preparing himself for jail after being found guilty in court.
He also offered the Mullen the family an apology for claiming the life of their loved one.
"Denis Mullen was an entirely innocent man," he said.
"It actually broke my heart when I realised I had I had killed an innocent man. He was a totally harmless being.
"I'm saying that on the record and I want Denise and her mother to know that."
"I can't go into detail, but although I killed Denis Mullen, other people were responsible.
"I took to the drink - I was drunk every night of the week when I found out I'd killed an innocent man - before I went to jail.
"I was used. But I'm really sorry I did it," he said.
Denis Mullen was shot dead on the doorstep of his family home at Collegelands near the Moy in Co Tyrone on September 1 1975.
Beattie also fired 13 machine-gun bullets at his wife Olive, who was forced to flee by jumping out a window.
Four-year-old Denise Mullen was found in her blood-splattered nightie trying to revive her murdered father.
Speaking from the art studio in his home - his paintings are regularly sold in top auction houses in Belfast - Beattie (65) also revealed he is now fearful for his life after receiving what he believed was a sinister warning when he was out on his push bike near his home last week.
And he firmly believes he is being targeted by British secret service agents who believe he may be about to spill the beans on the close connection between the military intelligence and loyalist killer gangs in mid-Ulster.
Said Beattie: "Without doubt this was a warning. It's how the spooks operate.
"I was out on my bike for a bit of exercise. I was coming up a long slow hill. Suddenly a van appeared over the brow of the hill. I know every vehicle around here, but I'd never seen this one before.
"I couldn't believe it when I saw it come over the white line in my direction. I braked on the bike because I thought he was going to knock me off the road.
"At the very last minute, the van swerved off and as it did so, the driver waved at me," said Beattie.
"It definitely was a serious warning. I tried to report it to the police, but I couldn't get through."
When asked to elaborate on connections between the UVF and military intelligence during his days as a loyalist paramilitary, he said: "I didn't realise it at the time, but looking back, I was being controlled by a serving member of the British Army, who was clearly an intelligence officer.
"Maybe the spooks think I know more than I actually do. Maybe they think I know more about the 'Dirty War'.
"I covered up for this man at my trial, but please believe me, I'm not covering up for anyone this time."
As he readied himself for jail, Beattie also said he was more than prepared to meet his maker.
"I don't fear death and I don't fear jail," he said.
"I made my peace with God on the 17th of January 1981. I don't care if people forgive or not. I've made my peace.
"Some people would like to see me hung by the neck and they still wouldn't be happy," he said.
Councillor Denise Mullen last night said she was more than happy with the outcome of the court proceedings in Dungannon earlier this week.
And she revealed it has now emerged her Dublin-born father Denis was fearful for his family years before he was murdered.
"One of daddy's friends in Wicklow told how he had written to him asking to look for a house for us south of the border because he feared thing were going to get worse," she said.
"How right he was."