Hmm, Ambushes on honourable foes are certainly out - but what about foes deemed 'beyond the pale' or 'beneath honour'.
So, for example, is trapping/ambushing 'mindless' creatures ok, is ambushing undead/demons ok? Neither is even remotely going to come under the honourable enemy condition and both are generally considered destroyed/exterminated rather than murdered or even killed.
As a paladin I figure Robhan is at the ultimate extreme - if the enemy can think and talk he'll be openly challenging it like death-wishes are in fashion again, however historically once a crusade had been declared, excommunication levied, person was declared outlaw, then the usual civilised defenses they had against 'dishonourable combat' simply vanished.
A bit like the Geneva convention - if your enemy wears a uniform then they had numerous rights, if the enemy went mufti as spies then the gloves come off. Similarly for armies which refused to wear uniforms - the protections their civilians had under the convention are seriously weakened.