"To them, vengeance is a code of ethics that is acceptable."
This seems all wrong to me!
Stevenson takes a very sympathetic approach with the Highlanders. He wants us to LIKE them. He would not have considered vengeance an acceptable code of ethics, so he would not have meant for us to view the Highlanders as a vengeful people. There is something missing in this Sparknotes interpretation.
Considering the historical context, we know the Highlanders considered the English Whigs to be USURPERS. Therefore, they did not view the English as having a legitimate claim to the throne or having any legitimate authority over them. The Highlanders were not acting out of vengeance! They were acting out of LOYALTY to what they believed was the rightful government. David, being English, did not understand that loyalty so the Highlanders appeared insubordinate to him at first. Stevenson is asking David and his reader to look at the situation from a different point of view.
Like this: If the USA were suddenly invaded and taken over by a foreign country, would YOU feel any loyalty to the forcibly installed government? Or would you continue to view the US government, even in exile, as your legitimate authority? I'm betting you would not feel the slightest interest in supporting an invading government, but you would not be considered VENGEFUL. You would considered a loyal patriot.