Ilgamuth was a little uncertain about Rabadash from their first meeting, but yeah, breaking his sworn oath is a huge move. Even just letting Malindra and Shezan plot while doing nothing to either help or hinder them would have been difficult. Actively working against his lord... that's drastic. But he has moral lines in the sand past which he can't continue supporting Rabadash, and in this scenario, they get crossed with extreme prejudice.

He deals with the whole situation very badly, emotionally speaking, and never feels properly honorable again. Of course, Shezan is suffering her own crisis over betraying and working against her might-as-well-be-brother, so they have that in common... which actually delays their eventual romance, because they remind each other of their own shortcomings.

Malindra Takhun is going to have (I hope) a larger role in the sequel to "Out of Season" and HHB that I will write one of these days. That story is nothing but politics and people dealing badly with massive life changes. And then more politics, because I can. *evil smile*