I don't quite see this.
Why would the murder be objectionable? Clearly, K'Ehleyr put up a fight. She lost, so it was her own damn fault for being an inferior Klingon. This still doesn't mean she goes to Hell; she just gets to boast a bit less in the feasts of afterlife.
Dealing with a political opponent by gutting her sounds like a very Klingon thing to do, much superior to Gowron's clumsy attempts at bribery. If anything, K'Ehleyr's supposed attempt to wield information as a weapon would be less Klingon than Duras' more straightforward response.
Whatever Duras was, "coward" doesn't seem to enter it. We have no reason to think the winner of the duel with K'Ehleyr would have gone unidentified, or indeed that Duras would have made the slightest effort to hide his role there (it's not as if Starfleet starship quarters have a "fight in progress" alarm in any other adventure, either). Duras also willingly enters a duel with Worf, much like Gowron later on, when his status (as a leader of greater stature than Worf) and practical circumstance (as the commander of many an armed man in the room) would allow him to forgo that.
Treachery has never been identified as a vice in Klingons. That is, Klingons engage in it a lot, in basically all Trek eras, but when they do so, this is basically hailed as a virtue instead, such as with Koloth's clever poisoning scheme. Plots to outwit the enemy are pure warrior stuff, at least as long as there's some clanging of swords involved, too.
What is frowned upon is allying with the enemy, but I guess Duras would have the advantage there: even if caught, he's merely proven equal to Gowron, not inferior...
(As for witnesses, I guess this meant Duras' men.)
Timo Saloniemi
