How we cite our quotes: (Chapter.Page)
Quote #1
Yseut realized that Brangain constituted a potential danger, since she alone could betray the lovers to Mark. For her own safety, Yseut decided to have Brangain killed by two of her servants. (1.45)
Fearing Brangain's betrayal, Yseut betrays her first. She has no reason to believe her trusted servant (who, by the way, obligingly took Yseut's place on her wedding night) will turn on her but decides to have her killed anyway. Yseut doesn't come out looking too great in this episode. The story may be trying to make a point about how the paranoia surrounding adulterous love causes people to do things they might otherwise regret.
Quote #2
'Fair maid, I have good reason to be sad and anxious. Brangain, I will tell you everything. I do not know who wanted to betray us today, but King Mark was in the tree by the marble block.' (2.54)
Here, Yseut uses the word "betrayal" in the sense of an exposure or revealing, a meaning it will often have in Tristan. Of course, following the values of the story, the greatest of which is love, doing anything to prevent or thwart love does represent a betrayal in the other sense as well.
Quote #3
'My lords, you are loyal to me. As God is my help, I marvel that my nephew should have sought my shame; but he has served me in a strange way. Give me your advice, I beg you. You must advise me well, for I do not want to lose your service.' (3.61)
By saying that Tristan has "served" him in a strange way, Mark uses the language of feudalism, in which a vassal serves his lord, to indicate that Tristan's shaming of him is also a betrayal of his lord. He contrasts Tristan's betrayal with the loyalty of his barons, showing he trusts their loyalty implicitly by asking for their advice and "service."