Oddly enough, given all of the political reasons for killing off various Douglases, the most I can find concerning David Barclay's killing of John Douglas is that is was a "private quarrel."
https://www.google.com/books/edition/Publications_of_the_Scottish_H...
But thanks for the question! I think I will add this series of assassinations to the list of bad behaviours we talk about on True Crime Medieval.
lol
(truecrimemedieval.com -- https://www.geni.com/projects/True-Crime-Medieval/895188)
http://www.venitap.com/Genealogy/WebCards/ps37/ps37_310.html
Source: THE SCOTS PEERAGE, ed. by Sir James Balfour Paul, Vol II, Edinburgh, 1906, pp.222-3.
Notes for Lord David (1) BARCLAY Of Brechin
He received from King Edward I a grant of the lands of Avoch in Ross-shire, but about 1307 Hugh Ross, son of the Earl of Ross, petitioned Edward II that the lands might revert to him as Sir David Barclay had joined Bruce. This corroborates Barbour, who implies that Barclay had joined Bruce before the battle of Methven, and he was apparently taken prisoner there, on 19 June 1306.
Nothing further is known for some years, and he may have remained a captive until after Bannockburn, as the next notice of him is on 26 March 1315, when he granted certain lands to his future wife.
After the death of his father-in-law, Sir David Brechin, in 1320, Sir David Barclay received from King Robert Bruce a grant of his forfeited lands of Rothiemay, Brechin, Kinloch, and others. Henceforth he appears as Lord of Brechin.
In 1327 and 1329 he was one of the Auditors of Exchequer, and he was Sheriff of Fife in 1328. He acted as Steward of the Household of the young Earl of Carrick, afterwards David II, and he had superintendence of the burial arrangement of King Robert Bruce at Dunfermline in 1329.
In 1342, he, by command of King David II, seized and imprisoned in Lochindorb an ambitious churchman, William Bullok, who had fallen under the King’s displeasure.
Some years later, probably on account of some feud, or perhaps in revenge for the death of Sir Alexander Ramsay, he procured the death of John Douglas, brother of Sir William Douglas, the ‘Knight of Liddesdale,’ at a place called Forgywood.
In revenge, he was himself assassinated by hired assassins at Aberdeen, on 25 January 1350, under circumstances of cruelty and treachery.
http://drcallumwatson.blogspot.com/2019/06/rough-justice-arrest-of-...
Rough Justice: The Arrest of Sir Alexander Ramsay of Dalhousie, 1342
On this day in 1342, Sir Alexander Ramsay, recently-appointed sheriff of Roxburgh, was presiding a sheriff court at Hawick. In the midst of the proceedings, Sir William Douglas of Liddesdale arrived with a band of armed men, who seized Ramsay and dragged him back to Liddesdale's castle at Hermitage where Ramsay eventually starved to death. Today's blog post will look at the causes of this extraordinary incident, its consequences, and what these things tell us about the state of things in Scotland in the early 1340s.
School of Hard Knocks: Scotland in the 1330s and 1340s ....
Thanks Anne Brannen and Erica Howton and all. Sir John Douglas of Lothian is my 19th ggf or something or other and my father bears the "Douglas" as his middle name for his Scots lines. Interesting history every day on Geni!