Kingsmills was a brutal horrific slaying of innocents, it did not however happen in isolation. The day before it happened Six Catholics from two families were killed. Many from both sides in months previous had been killed.

Three members of the Reavey family were shot at their home in Whitecross and four members of the O'Dowd family were shot at their home in Ballydougan.[1] Two of the Reaveys and three of the O'Dowds were killed outright, with the third Reavey victim dying of brain hemorrhage almost a month later.^Source

In this period of our history, they would be referred to as tit for tat killings. The background which involves the British state in collusion with Loyalist Paramiltiaries, the Glennane Gang, which included serving RUC and British Soldiers.

On 10 February 1975, the Provisional IRA and British government entered into a truce and restarted negotiations. The IRA agreed to halt attacks on the British security forces, and the security forces mostly ended its raids and searches.[8] However, there were dissenters on both sides. Some Provisionals wanted no part of the truce, while British commanders resented being told to stop their operations against the IRA just when—they claimed—they had the Provisionals on the run.[8] The security forces boosted their intelligence offensive during the truce and thoroughly infiltrated the IRA.[8]

There was a rise in sectarian killings during the truce, which 'officially' lasted until February 1976. Loyalists, fearing they were about to be forsaken by the British government and forced into a united Ireland,[9] increased their attacks on Irish Catholics/nationalists. Loyalists killed 120 Catholics in 1975, the vast majority civilians.[10] They hoped to force the IRA to retaliate and thus end the truce.[10] Some IRA units concentrated on tackling the loyalists. The fall-off of regular operations had caused unruliness within the IRA and some members, with or without permission from higher up, engaged in tit-for-tat killings.[8] Irish National Liberation Army (INLA) members, and current or former members of the Official IRA, were also involved.[8]

Between the beginning of the truce (10 February 1975) and the Kingsmill massacre, loyalist paramilitaries killed 35 Catholic civilians in County Armagh or on its borders.[11][12] In that same period, republican paramilitaries killed 16 Protestant civilians and 17 members of the security forces in the same area.[11] Many of the loyalist attacks have been linked to the Glenanne gang; a secret alliance of loyalist militants, British soldiers from the Ulster Defence Regiment (UDR), and police officers from the Royal Ulster Constabulary (RUC). A former member of the group said they wanted to provoke a civil war, believing that when civil war erupted they could then "crush the other side"^source

In the aftermath of Kingsmills the Glennanne Gang sought to escalate by massacring up to 30 schoolchildren in the village of Belleek.