I think by 'honourable' most people mean he was one of the good guys. He wasn't a psychopath or corrupted by power. He was doing the right thing. He was his father's son. He was driven by vengeance above all else, but I think there is honour in that too. Avenging the murder of his father, members of his family, his friends and people, the destruction of his home and title. It's a tragic course of action, but understandable. It's something you can get behind and root for. Fist pump the air when it works out for him. I like to see that rewarded in fiction. There is profound emotional payoff in vengeance. It's satisfying. Of course, in real life vengeance doesn't help, just fucks you up more. But in fiction, it's a classic mythical trope. An appealing one.
I understand your point. The definition differs a bit in the context of the story, though. Like when you compare Robb to his father. Eddard would have done the "honourable" thing by sticking to the arranged marriage (which he did) at the detriment of the woman he loved (happened in the book, not sure if she'll get a mention in the series, and no, it's not Jon Snow's mother.) What annoys me, though, is that Robb in the book actually did the honourable thing. I'm going to spoil the books a little tiny bit here, but it's doesn't matter because the story has advanced: in the book, Robb hears about his brothers' supposed death (Bran and Rickon) and is so devastated about it that he seeks comfort with a noble girl and basically deflowers her. After that he's like "shit, I'm promised to another.... BUT I can't leave the girl like this, I'm honour-bound to marry her!" so he does. Not that he didn't love the girl, you know, but he did it for the sake of that land and those times' honour code. That's what they didn't show in the series. He's trying to be his father's son, not just following his heart and lust.
And technically his quest for vengeance, while awesome and badass, is rebellion against his rightful kind
(they don't know about the twincest)
Topic: Game of Thrones