“Then Cú Chulain began to grieve and lament for Fer Diad, and spoke these
words: ‘Ah, Fer Diad, it was a sorry thing that you didn’t speak with those who knew my high, brave deeds, before you came to fight me. ”
Cú Chulainn speaks of Fer Diad like a brother, yet he himself is the one who killed Fer Diad, and in a rather horrific fashion using the Gae Bolga. There seems to be a sort of disconnect between what is done on the battlefield and what the warriors think to themselves personally, since Cú Chulainn didn’t seem all too broken up about using his secret technique in battle, but as soon as it was over he shows signs of grief at Fer Diad’s death. It could also be that Cú Chulainn sees Fer Diad’s death as tragic, but altogether necessary. After all his death would protect both Ulster and Cú Chulainn’s honor, so in a certain sense he might simply see it as a price he has to pay for what needs to be done.
I think that this shows as well, to a good extent, the degree to which familial and interpersonal relationships are sacrificed for the ulterior motives of clan welfare and wealth. In the Tain, we see so many examples of characters almost losing a sense of individual identity, going to extreme lengths to do what is best for the collective that they are a part of. There is a strange contrast between a desire for individual glory and happiness and a very diminished sense of self worth among characters at the same time.
I agree there is a disconnect between Cú Chulainn himself and himself as a warrior. This is seen in Cú Chulainn as well as Fer Diad before the fight when they displayed conflict within themselves for fighting each other after being foster brothers. Their ways of war is put above this notion of family because of the circumstances put onto them. All characters against Cú Chulainn broke fair fight, including himself, to try to fulfill the necessary deed of ending a threat. One´s honor and overall feelings against such acts are to be ignored to fulfill their purpose as pawns in war.