The context of killing

The murder of John Comyn of Badenoch, and his uncle Robert, in the church at Dumfries on 10 February 1306 was one of the most shocking acts of the period. Less clear is the motive. 

If we take a look at the wider context of the killing, especially the immediate aftermath, the fog starts to clear. Once Bruce and his men had finished off Comyn, they rode to Dumfries castle and seized it from the sheriff, Sir Richard Siward. They were joined by Sir Roger Kirkpatrick who, somewhat ironically, had been holding court nearby as one of Edward I's justiciars of Galloway. His fellow justice, Sir Walter Burghdon, was then taken prisoner along with the other English officials in the region. 

The intrepid band then rode to Siward's castle of Tibbers, seventeen miles north-west of Dumfries, and seized it along with Comyn's castle at Dalswinton. The royal castle at Ayr soon followed, as well as James the Steward's castle at Inverkip and Rothesay castle on the Island of Bute. Later evidence strongly suggests that the royal castle of Dunaverty on Kintyre and Bruce's castle of Loch Doon in Carrick were provisioned at about the time as the murders at Dumfries. 

In short, Bruce had taken a string of strategically important castles that enabled him to control the sea-routes in and out of western Scotland. His actions, and those of his supporters, were efficient and co-ordinated, implying their strategy had been carefully worked out beforehand.  

This was emphatically not the behaviour of a man who had murdered Comyn in a fit of rage or panic, on the spur of the moment. Rather, Comyn and his uncle had been carefully targeted, as part of Bruce's long-planned seizure of power. Now he had to wait for the reaction of Edward I, slowly expiring in distant Westminster. But that's another tale... 

Sources: 

Traitor, Outlaw, King Part One: The Making of Robert Bruce by Fiona Watson 

Edward I by Michael Prestwich 

Disunited Kingdoms: People and Politics in the British Isles 1280-1460 by Michael Brown