Robert Boyer, 43, was suffering delusions when he carried out the "ruthlessly executed" attack last July on Keith Frogson, 62, a National Union of Mineworkers activist, outside his home in the pit village of Annesley Woodhouse, Nottinghamshire, a judge at Nottingham Crown Court was told.
Two weeks after Mr Frogson was shot and clubbed with the crossbow until it was clear he was dead, his daughter Rachel was almost killed when Boyer set fire to the family's house - which was 50 yards from his own. She and her partner, Dilshard Junaideen, both 33 and who now live in Sri Lanka, fled for their lives.
Mr Justice Pitchers adjourned the case for 12 weeks for further psychiatric reports and an assessment on Boyer, who is being held at Rampton high security hospital in Nottinghamshire.
Andrew Easteal, for the prosecution, said the defendant had convinced himself wrongly that Mr Frogson was intent on dismantling his house brick by brick, that acid was being thrown at the brickwork and that a screwdriver had been used to chip away the bricks. "Mr Frogson was completely innocent of this and had no idea what Boyer was thinking or the delusions he was suffering," he said.
Mr Easteal said Boyer's obsession was not linked to Mr Frogson's position on the opposite side of the picket lines in Nottinghamshire, where the formation of the breakaway Union of Democratic Mineworkers (UDM) was responsible for some of the most bitter scenes of the strike.
Nottinghamshire Police's investigation of this possible motive was prompted by Mr Frogson's defiant union role in the 1980s when all but 52 of Annesley's 800 men abandoned the NUM. He would shout "You scab bastard" across the street to any former miner he knew to be UDM. The union's respect for him was illustrated when 1,000 people, including Arthur Scargill, its honorary president, attended his funeral last August.
By contrast, Boyer was a solitary man who struggled for an identity when the pits closed. He was 23 at the time and apart from spells as a security guard and a tin production worker, he remained unemployed. He was divorced years ago and has no children. Neighbours remember him working with a blow torch on cars which he kept in the house and shooting at birds with a rifle from the window of his terraced house. He had convictions for shoplifting and neighbours say that he was challenged after a neighbour caught him siphoning petrol from a car belonging to Mr Frogson's daughter Mandy.
This may have contributed to a lingering personal dispute with Mr Frogson, who was always a stickler for an old coal mining community code of sorting out criminals without recourse to the police.
In the weeks after the killing, Boyer went on the run and hid in a part of Sherwood Forest known as Robin Hood's Hill. A survival fanatic, he constructed a makeshift shelter of bracken and foliage where he stayed for more than three weeks. Officers found the hideout dug into a hillside and captured him with the use of a stun gun.
Rachel Frogson, her sister Mandy, 36, and her brother, Wayne, 34, were all in court to see Boyer, 43, plead guilty to manslaughter and arson being reckless as to whether life was endangered. The Crown Prosecution Service said: "It was only after careful consideration and consultation that the prosecution team decided the evidence of Boyer's mental condition was such that a charge of murder was not sustainable."Reuse content