If you admire Caeser, you might admire some of his noble traits yet the man in the end killed a lot of gauls for his personal gain, pretty evil stuff.
Good example - the Gauls weren't 'civilised' and so were not included within the value set of the Roman honour system. By contrast the Romans regarded the Greeks as being within their value set, ie suitable subjects for honorable behaviour. Yet the Greeks from the POV of their own code saw the Romans as dishonourable because eg the Romans wouldn't accept battlefield surrender. In the Greek system, when a warrior surrendered you took him prisoner. In the Roman system, when an enemy stopped fighting you stabbed him in the gut and went on to the next one.
Different, incompatible honour codes can create very interesting dynamics for an RPG. For instance, both sides initially treat the other 'honourably', but then one does something -within their code - the other regards as an atrocity. The normal response is to then exclude that foe from the value set and engage in all-out war. But there's also the possibility of trying to clear up the mistake, establish new & mutually acceptable ground rules, etc.
One side's "execution of enemy combatants not in uniform" is another's "massacre of civilians".