What passed between them is not known with certainty; but they quarrelled, either concerning their mutual pretensions to the crown, or because Comyn refused to join Bruce in the proposed insurrection against the English; or, as many writers say, because Bruce charged Comyn with having betrayed to the English his purpose of rising up against King Edward.

On the eighteenth of May he was excommunicated by the Pope, on account of the murder of Comyn within consecrated ground, a sentence which excluded him from all benefits of religion, and authorized any one to kill him.