I allways thought this is common mentality among Githyanki ...
I mean "to become Queens chosen" being something like ultimate goal for everyone.
Ofc, it means for them that they will be killed and their soul eaten by her in order to raise her power even futher ... but they dont know that. 
As for Kithrak betrayal ...
Lae'zel seems to believe that "protokol" she is talking about, orders everyone to help infected Githyanki ... sadly (for us at least), Kithrak seems to believe that those who were infected should be simply eliminated ...
And since in Lae'zels loyal, but honestly quite simple mind, there is no way that Vlaakith would do any harm to her people ... notice that Lae'zel dont talk about Githyanki as Vlaakiths people, nor even as her soldiers (wich they most likely are for her) but as her "childern" (so she either actualy is royal blood, or she just see Vlaakith as benevolent and loving ruler) ...
Therefore his betrayal is the only explanation she is willing to accept.
//Edit:
party banter where she literally says murder is good because it culls the weak.
There is even another where she say that she herself killed several of her cousins during training ...
Thats just it, in Githyanki eyes (as far as i understands it) if you are weak, you have no right to live, none of that is murder for them therefore ... but Lae'zel dont concider herself weak ... and so she dont apply this rule to herself.
