Here's the most detailed version of the story I can find with Google Books, from an article, "Gleanings in Family History from the Antrim Coast: The Mac Naghtens and Mac Neills" (pages 127-144) by George Hill, in Ulster Journal of Archaeology, Volume 8 (Belfast: Archer & Sons, 1860), page 134-5:

[John MacNaghten] died, when his son and successor, John MacNaghten, was only a child six years old. The career of this son was melancholy, and his fate appalling. He was born about the year 1722, and educated first at Raphoe, and afterwards in Trinity College, Dublin. Even while attending school he became addicted to gambling, and continued a slave to that vice until it finally led to his ruin. He was compelled, when very young, to sell a part of his estate and mortgage the remainder; in order to meet his gambling debts. His first wife was a daughter of Dean Daniel, and sister to Lady Masscreene. Her husband's reckless conduct was the cause of her death,—an event, however, which he sincerely deplored. His affairs soon after became desperate; but he still had influential friends who pitied him and helped him. Lord Massereene obtained for him the appointment of collector of taxes for the County of Coleraine, worth upwards of £200 a-year; and Mr. Workman, who had married his sister, became his security in a bond of £2,000. In less than two years he lost this situation, having embezzled £800 of the public money. ln an evil hour, Andrew Knox, Esq., of Prehen, near Derry, invited the now friendless MacNaghten to spend a few weeks at his house, until some other situation might offer. He instantly formed the design of marrying Miss Knox, a girl of only fifteen years of age, but an heiress in her own right. MacNaghten induced her to read over with him the marriage ceremony in the presence of a third person, and then claimed her as his wife. Her father of course, resisted, and finally set aside the claim in the Court of Delegates. When Miss Knox was afterwards being removed to Dublin, accompanied by her father and mother, MacNaghten, with a servant and two tenants, surrounded the carriage on the road, about three miles from Strabane, for the purpose, as he alleged, of rescuing his wife. Mr. Knox was attended by two or three men servants, well armed, and a scuffle instantly ensued on the carriage being stopped. Several shots were fired by both parties. MacNaghten, having been wounded in the back, came forward and fired deliberately into the carriage, with the intention of shooting Mr. Knox. The contents of the gun, however, entered Miss Knox's side, and she died after a few hours of agony, during which she uttered no complaints against any one, and only prayed fervently to be released from suffering. This melancholy affair occurred on the 10th of November, 1760. The names of MacNaghten's three associates were, George McDougall, James McCarrell, and Thomas Dunlap. Two hours after the murder, MacNaghten was taken after a fierce struggle, in which he first endeavoured to shoot his captors and then himself. McDougall and McCarrell escaped, but Dunlap was caught in a house at Ballyboggy, near Benvarden. He and his master were imprisoned in Lifford jail until the 11th of the following December, when they were both tried, found guilty of the murder, and sentenced to death. When sentence was pronounced, MacNaghten implored the judges to have mercy on Dunlap, whom he spoke of as "a poor, simple fellow, his tenant, and not guilty of any crime." MacNaghten's defence of himself at the trial drew tears from many eyes; and his general deportment afterwards was such as to make him an object of interest to the people of the town and neighbourhood of Lifford. No carpenter could be found to erect the gallows, and an uncle of Miss Knox, with the assistance of some friends, was obliged to provide one, rather than see the criminals hanged from a tree; the smith who knocked off the hand-cuffs from MacNaghten, as a preliminary to the execution required by law, did so under compulsion; and the hangman had to be brought all the way from Cavan. MacNaghten conducted himself with the greatest coolness and dignity, declaring repeatedly that the anticipation of death was much more dreadful than the reality. To make his exit as easy and speedy as possible, he adjusted the rope securely on his own neck, and ascended to the very top of the ladder before throwing himself off, that the struggle might thus be terminated in a moment. The rope broke! The immense crowd uttered a triumphant shout, and urged him to escape, making way for him in all directions. But no. He calmly remounted the ladder, remarking, as tradition affirms, that no one would ever have to point at him or speak of him as half-hanged MacNaghten. The rope was knotted and adjusted as before, and after having done MacNaghten to death, it was removed to perform the same office for his wretched tenant and associate in crime. Their bodies were buried in one grave, behind the church of Strabane.

On the death of John MacNaghten, who left no children, the Benvarden property was sold, and passed out of the family.