Duncan and Royland - Here's Why
I'm seeing a lot of people unhappy with the way the traitor situation plays out. That's pretty understandable, and I haven't played it both ways yet, but something that came across in my playthrough seems like a key fact to the whole matter:
Duncan and Royland, whoever the traitor is - they're not true traitors. They're extreme loyalists. They don't want to see the house destroyed. The one who's NOT picked as Sentinel acts out because they see no other way to save the house. Without the title of Sentinel, they feel like they're powerless to influence Rodrick, so they back the Whitehills so they can end the conflict as quickly as possible, with as few deaths as possible. Granted, I think it makes a little less sense for Royland to think this way. But I don't think it's quite as out of character as some seem to think.
Duncan is extremely diplomatic; no matter what you do to make peace, it's not enough for him. Royland is the opposite, extremely aggressive. They've been this way since the beginning, always searching for the most extreme answers to every problem. I think it makes sense that without something (like the title of Sentinel) to placate them, they'd find a way to rebel against a leader that defies them.
It's not betrayal for them; I think they're both acting out of warped sense of the greater good.
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But you can still take their advice and do everything they suggest, even if you didn't pick them as the Sentinel. And they'll still betray you, even if you take their advice ever step of the way. It all comes down to having a hissy fit over not being the one to wear the shiny bracelet. Anything they say about trying to protect the House falls flat when you realize that you've done everything they wanted you to, and they're still pissed.
This. I was watching a youtube video where Duncan was the traitor in this one guy's playthrough, and all he said were things that weren't even determinant like "You let Arthur die," and "Ethan was just as narrow minded as you." Lazy writing is all it is.
Their main argument is always "You're just as X as Ethan was" which is, in fact, BS. It's determined by who ETHAN picked as Sentinel, so even if, while playing as Rodrik, you do a total personality 180 and do everything Advisor A tells you to, Advisor A will still call you an idiot for not doing what they told you to, because your dead brother gave a shiny glove to Advisor B.