Well, at least we know some example what doesn't: Blowing someone's ship up by sabotage, or using poisoned food.
Attacking from behind isn't necessarily dishonorable, since a good sneak attack shows a hunter's skill, but it may be frowned upon if the enemy is a warrior.
So it also depends on who is the enemy. If I recall correctly we hear about it in the Heart of the Tiger novel. Melek is shocked by Thrakhath's plan with the bioweapons because that is not the way a warrior should behave, and Thrakhath explains to him that he doesn't consider it dishonorable because the humans are no warriors but prey animals.
And if you think about it: That's just the way we see it as well: Stabbing or shooting people in the back is not considered to be honorable, but nobody complains about a huntsman shooting a deer or boar from far away and possibly from behind.
So Thrakhath's murdering his way to the throne may for example mean the following: Imagine another Kilrathi that is of imperial blood is born. Thrakhath fears he might be dangerous for him one day, so he waits until the other Kilrathi is just old enough to be challenged, knowing that it is far less likely that a young Kilrathi beats him (who is a veteran already) in a duel.
Then he finds a reason to be offended by him and challenges him to a duel. He kills him and it is honorable because the younger Kilrathi (theoretically) had a chance of winning. Still from our point of view this is just murder.
He may also transfer him to a "very honorable post" (which means near the front line, hoping he is killed). Isn't there even a story in the bible where King David or so kills a hot woman's husband like that? (I'm not a bible expert but I think I remember the story) It is basically the same thing. He didn't murder him in person but letting him fight in the first line of battle is as good as a death sentence, especially if Thrakhath knows that he isn't a particularly good pilot.