After the Battle of the Trident, Robert Baratheon, Eddard Stark and others had a decision on their hands. Barristan the Bold, one of the greatest knights in the realm, lay badly wounded after suffering 3 or more wounds during the fighting. Roose Bolton counseled his throat be cut (this is also the very first time Roose’s name ever appears in ASOIAF). However, Robert would do no such thing. As he did with so many others, he flipped a foe into a friend.
Robert, still young and uncorrupted by the throne, was not yet a man willing to murder. He believed that the only place for killing an enemy was on a battlefield, save perhaps an arranged duel or the like. Robert said (according to Ned):
“I will not kill a man for loyalty, nor for fighting well”
It was not necessary for anyone on the side of mercy to argue, since Robert was already set on allowing Barristan Selmy to live. But had he been on the fence, or intent on an execution, any student of Westerosi history could’ve made a great point that may have swayed Robert on a personal level. A particularly glorious deed of Ser Barristan’s looms large here… One very relevant to House Baratheon.
Barristan the Bold is famous for many reasons, but aside from Duskendale, the slaying of Maelys Blackfyre is surely at the top of the list. Why would this matter to Robert? Because Maelys the Monstrous himself slew Robert’s own grandfather, Hand of the King and Lord of Storm’s End, Ormund Baratheon, during the War of the Ninepenny Kings. Robert had not even been born by this point, but surely he heard how his Lord grandfather died. Robert is no student of history, but in a land where blood debts are common currency, this would surely argue strongly for a pardon.
In fact, perhaps this did factor in Robert’s decision to name Ser Barristan the Lord Commander after the war.