First of all, I'm pleased and proud that you were reading
The Spoiled Plains!!!!
Well, let me try to give my chronology of the end of the Bloodguard. ("BG" makes me think Bene Gesserit.

)
The first thing to go wrong was Kevin ordering them away so that he could invoke the Ritual of Desecration. They failed their Vow. Yes, they were tricked into failure, but they shouldn't have allowed themselves to be tricked. They should have said, "No, we're staying with you. We Vowed to protect you, and we can't do that if we leave you alone."
Next, because they revealed the name of the 7th Ward, Elena died. They didn't just fail to protect a Lord, a Lord died because of something they did!
Finally, they fought on the side of Corruption. No matter how it happened, no matter the strength of the Illearth Stone, three of them went
against the Vow, killing many people, intending to kill more - even Lords! They saw that their Vow was not absolute, that they could act against it. So they abandoned it.
With the
Haruchai, it's all or nothing. If they can't do something totally, absolutely, they don't do it at all.