And six years later - seconds before he was handed four life sentences - his voice boomed across the courtroom: "I'm being sent to prison because I wouldn't become an informer like the rest of them," he yelled at the trial judge.
Somerville was sent to jail for 35 years and, despite being firmly opposed to the Good Friday Agreement, he was released under its terms after spending just 18 years behind bars.
A Sunday World investigation into the Miami Showband atrocity has revealed the now deceased killer's assertion was true.
Somerville would never have gone to prison if he'd agreed to work for the RUC.
Somerville told other paramilitary prisoners how detectives repeatedly tried to persuade him to become a Special Branch tout operating inside the UVF.
"John said the cops told him there was no need for him to go to prison. Others had already been convicted for the Miami attack and they wanted John to stay in the UVF, but to work for them," said a former loyalist prisoner who served time in prison with Somerville.
"They also hoped he would one day take over the leadership of the organisation in mid-Ulster when Robin 'The Jackal' Jackson either stood down or was executed."
Our source also claimed John Somerville told him that in an attempt to break him, a police officer entered the interrogation room carrying a severed human arm in a plastic bag.
"The arm belonged to John's brother Wesley, who was killed in the Miami explosion. It had been blown off his torso when the bomb prematurely exploded as a result of static electricity.
"The cops showed John a bag containing a human arm with a Mid-Ulster UVF tattoo on it. They asked him if he recognised it or could he identify it. It was his brother Wesley's arm," said the source.
Findings in a report carried out by the PSNI's Historic Enquiries Team into the Miami atrocity stated that there was fingerprint evidence linking Robin Jackson to the attack.
But it went nowhere when a senior RUC officer advised the UVF leader to lie low for a while.
Jackson was never charged with the Miami atrocity. But two other UVF men, Thomas Crozier and James McDowell - both soldiers in the UDR - were jailed for life.
Robin Jackson died of cancer in May 1998 aged 49.
But our investigations this week revealed that shortly before he died, Jackson sent for self-confessed loyalist gunrunner Willie Frazer, also now deceased.
Jackson informed Frazer that John Somerville was due for release. And he wanted Frazer to ensure the Miami killer received any assistance he needed in adjusting to life on the outside.
"Robin Jackson and John Somerville had been very close friends since joining the Mid-Ulster UVF. They had killed many Catholics together and they trusted each other implicitly," said our source.
Days before the Miami attack, Robin Jackson murdered William Hanna, the UVF commander in mid-Ulster.
Jackson was convinced Hanna was a Special Branch informer and he feared he may spill details of the imminent Miami attack to his RUC handler.
But the Sunday World has also learned that on January 4 1976, Jackson was accompanied by John Somerville when he burst into the home of the O'Dowd family at Ballyduggan, near Gilford, shooting three of them dead.
Twenty minutes before - in what was clearly a co-ordinated operation - another loyalist killer gang made up of police officers and UDR soldiers launched a murder attack on the Reavey family home in Whitecross.
Gaelic footballer brothers John Martin Reavey (24) and Brian (22) died at the scene, while 17-year-old Anthony died three weeks later.
John Somerville was born in 1940 into a respectable small farming family from outside Moygashel, Co Tyrone.
As a teenager with strongly held anti-Catholic views, Somerville joined the B Specials, but he later went to sea as a member of the Merchant Navy.
But he quit when the Troubles erupted on the streets of Belfast and Derry in the autumn of 1969. And despite being married with a family, he immersed himself in loyalist terrorism.
Lightly built and standing less than 5ft 7in, he wasn't a stereotypical loyalist killer.
But along with his friend Jackson, he helped build the UVF in mid-Ulster. They received the blessing of jailed UVF leader Gusty Spence, who was serving life for the Malvern Street murder and shootings in 1966.
The fledgling loyalist terror group didn't operate in established command areas and Somerville and Jackson were able to kill at will over a wide geographical area.
They sprung terror attacks in south Armagh, south Down, east Tyrone and even as far away as south Derry.
And nearer home, they carried out five operations in one day in the Moy and Stewartstown.
They were both present when the Miami Showband bomb exploded, but the shootings which followed seconds later - including the slaughter of Fran O'Toole - were mainly the work of John Somerville.
"After the explosion, the red mist came down and I went mad with a machine-gun," he once told a fellow loyalist prisoner.
On April 17, 1975, Somerville and Jackson blew up a Catholic-owned cottage which was being renovated at Killyliss between Dungannon and Ballygawley.
The explosion ripped through the building, killing 21-year-old married woman Marion Bowen, who was eight months pregnant at the time.
Her brothers Seamus and Michael also died in the attack, which was later claimed by the Protestant Action Force, a cover name for the Mid-Ulster UVF.
Despite severe burns sustained in the Miami attack, a month later Jackson was soon back on a murder mission with his trusted fellow killer John Somerville.
Using a similar terror technique, they set up another fake checkpoint at Cornalaght, Newtownhamilton. And after stopping GAA fans 22-year-old Colm McCartney and 32-year-old John Farmer, they shot them dead.
Two days before, Jackson and Somerville had carried out a bomb and gun attack at McGleenan's pub in Armagh, killing its owner Jack McGleenan and customers Patrick Hughes and Thomas Morris.
On October 23, 1975, Somerville and Jackson led a UVF team in the savage murders of Peter McKearney (63) and his wife Jane (58) at their home near the Moy.
Unlike Jackson, Somerville was arrested in the wake of the Miami atrocity, but he refused to make a statement and was released without charge.
He also took part in fatal bomb attacks at Kay's Tavern in Dundalk and Donnelly's Bar at Silverbridge, south Armagh.
But as the death toll mounted, so did Somerville's drinking. The ruthless UVF killer fell into a deep depression fuelled by alcohol.
In 1978, he became a born-again Christian. And two years later when he was scooped in an RUC raid in Dungannon on September 26, 1980, he made a detailed confession outlining his involvement in numerous UVF murders.
According to loyalists who knew him, Somerville turned down countless RUC Special Branch offers to work as a police agent. But by this time, he was ready to go to jail.
He was charged with the Miami Showband murders and the attempted murder of bass player Stephen Travers, as well as the murder of Patrick Falls in 1974.
He was jailed for 35 years.
In prison John Somerville lived a reclusive life. His cell was kept immaculately clean and visitors were required to remove their shoes before entering.
Despite his conversion to Christianity, Somerville remained virulently anti-Catholic and he strongly opposed the Good Friday Agreement. He was released in 1998.
After meeting a new woman, Somerville settled on the Shankill and for a while he worked in the Harland and Wolff shipyard. He was unimpressed by the UVF men he met in the loyalist stronghold.
"A hundred of your men haven't done what I've done," he told a UVF leader.
In his flat - which few people visited - Somerville kept a large photograph of Robin Jackson on the wall of his living room.
In 2005, Somerville attended a ceremony in Portadown honouring his dead brother Wesley.
Among the large crowd he spotted Billy McCaughey, a former police sergeant and convicted killer who had named his UVF accomplices to investigating detectives.
Somerville walked over to McCaughey saying: "Get back on the bus. Touts aren't welcome here."
In a rare interview with the BBC Spotlight programme before his death of kidney cancer in 2015, Somerville spoke only once and very briefly about the Miami Showband Massacre.
Somerville said: "I'm not going to make excuses for my past. I did what I did. No one coerced me. No one forced me. It was my own personal feelings and convictions at the time these things happened."
And he insisted: "The Lord has forgiven me."
It was a forgiveness that few of the relatives of his victims were willing to give.