So, I think ultimately here we cannot appeal to emotion. No, we need to put Freddy's brain to work here. More than anything, we have to address Garagrim's shame at not being there. Let's look at his own logic:
Okay, here's our vector for attack. He lost his cool and got himself injured, such that he was unable to help later.
AKA he was acting like a Slayer.
So we need to point this out to him. We need to point out that he was fulfilling his oath as a Slayer in that instance, that what he is saying is that he wishes he acted with prudence rather than recklessness, AKA he wishes he had acted like a king rather than a slayer.
And now, here's the trick, we praise him for it. At that moment, he was a Slayer and not a king. By his oath he was bound to be reckless. And no son of Ungrim, no descendant of Baragor would go against his oaths! And now he is free of the Slayer Oath, and what an amazing thing that is! For a living Slayer to be rightly and justly decreed to have fulfilled his oath, what wonders. The oaths of kingship now bind him, but he has the choice of whether to take up the Slayer Oath once more. And now we ask, had he the choice before, would he have wanted to have acted as a King or as a Slayer? And then we ask, how does he want his father to be remembered? As the Last Slayer King, the one who finally relieved the weight of his ancestors oaths in a Doom that is unlikely to ever be exceeded, or as A Slayer King?
So we place the weight of duty and history upon him, and ultimately ask whether he values the honour of his ancestors more, or his personal shame more.