The press clippings:
Exactly what did Cairns do that the Irgun decided to assassinate him (his headstone)?
Well, Zev Golan notes that Cairns was widely accused of using torture against members of Irgun, including Irgun leaders Benjamin Zaroni and Mordechai Pacho. See here, Case 18.
But if you look at the Hebrew book, Megillat Menachem (Scrolls of Menachem), 2008, starting on page 114, you can read what he did to Professor (eventually) Menachem Levin in June 1939.
He offered him water in a cup, brought it to his lips and then withdrew, numerous times. He was slapped repeatedly in the face. Anti-Semitic abuse was hurled at him and degrading words shouted. He was then, after refusing to talk, struck multiple times with fists, on his back, in his kidneys and stomach. The next morning he was removed from the holding cell, kicked and beaten and then laid on a table and his soles were burned with cigarettes. The burn marks remained with him until he died, 60 years later. On the third day, again he was taken to the torture room. Water was dripped on to his head in additional to the normal beatings. He was denied water and tried to lick the drops off his face. They then placed wooden sticks between his fingers and toes and squeezed. Then they crushed his testicles.
On the fourth day, he was dressed in civvies and brought before Cairns who asked how he was feeling. Levin replied, "I feel excellent, Your men dealt with me in the most capable methods of torture." Cairns replied, "I am sure that they treated you according to the rules of curtesy and nicety as a you are a gentleman."
Levin refused to answer, was taken to the torture room and whipped on the soles of his feet, the Turkish falaka. He, of course, fainted during most of the applications of pain and this went on for six full days altogther. Cairns threatened to kill him and Levin replied, "Mr. Cairns, I really don't know which of us will outlive the other." He was then lead of to solitary confinement. Two weeks later, he was brought out to see Dr. Judah Magnes who had brought along AD Goitein. Levin told him and showed him all that had happend, including his many wounds, some still open. Magnes came a week later with another lawyer, Asher Levitsky.
Levin recalls, p. 137, that when he heard the blast on August 26, he was sure Cairns was the target. After personally torturing Binyamin Zaroni, his fate was sealed.