It should be noted that orcish honor isn’t a concept against fighting or violence: it is not about respecting one’s conscience, or respecting universal human rights.
In fact, a honorable death is sought after by most orcs, with them believing that death during combat is actually the best death one can desire. As far as I can tell, the orcish concept of honor is not against war itself, but war conducted with unfair methods. Saurfang is against Sylvanas’ methods more than the war itself.
In many ways, I always envisioned Saurfang’s rejection of Sylvanas’s war as based on something more than honor, something such as a genuine refusal to embrace that kind of brutal cruelty that his people had already embraced in the past. He abhors the war, but his culture - the orcs, the Horde - always embraced it.
So how does an orc express this new acknowledgement, a moral dimension that entails a respect for conscious lives, and a hate against the horrors of war? Well, through more familiar terms: he recognizes that Sylvanas’ way of fighting is unfair, and thus the code of conduct of old [honor] can become an ally of the new persona within himself [universal morality]. He can denounce this war without having to embrace fully this greater dimension - which is likely connected to his sense of guilt during the first war.
Is he incoherent at the beginning? Yes, but it is because he is developing something within himself.
Thus, eventually, to describe this refusal to embrace Sylvanas’s Horde he uses the concept he has inherited from his people, that of honor.
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Concepts of honor and dishonor can become quite weird. During medioeval times there was a samurai ritual in which a samurai would test a new sword or a new technique on a random passenger on a road, killing them. It was unlawful, yet it is debatable if it was also considered dishonorable (according to some authors I have read, it was not).
As a social construct honor is not necessarily a force of good, for example it alone isn’t enough to acknowledge inalienable human rights to all. Today we would consider most wars wrong, or a necessary evil. In terms of honor, a war just doesn’t have to be dishonorable - it would simply demand that a war is fought fairly.
…judging by the ancient times standards, I’m not sure how much it truly mattered in wars and if so, to which extent, and I imagine it was more important inside a nation, in duels of honor, feudal ties with your lords, rather than between two factions at war.