How is betrayal shown in The Poisonwood Bible by Barbara Kingsolver?

There are numerous instances of betrayal in The Poisonwood Bible.  In many of them, the betrayal when examined turns out not to have been a betrayal after all.  Often it is better described as the act of a limited person, doing the best they could under difficult circumstances, which then caused damage to someone else.  However, examining the betrayal takes a long time - the whole book, in fact - and during that long time, the betrayal seem inexcusable. 
The clearest example of this is the scene when the driver ants come to the village.  These are ants that run over everything and bite anyone who is in their path.  When they come in a huge flow, in the middle of the night, the only way to escape them is to get across the river.  So the whole village is rushing to the river to get on boats.
The Price family, as is typical of them, don't stop to help one another.  They flee, each on their own, except for Orleanna, the mother, who carries out Ruth May, the littlest one. 
Adah, at age 14, is one of the twins. She is lame because of a birth defect, and can only walk dragging one leg.  She asks her mother for help, but their mother is carrying Ruth May, and the only way she can help Adah is to say "Come on" and turn and start off for the river.   As far as Adah is concerned, her mother has left her behind.  She gets knocked down in the crowd, trampled on, and could have died, but their Congolese friend Anatole (sent by Adah's twin Leah) comes and saves her.  
To Adah, this incident "marks my life's dark center."  For many years, she goes on thinking that her mother has rejected her.  She imagines that her mother thought her life less worthy of saving than Ruth May's because Adah was an awkward, crippled teenager rather than a sweet little blond girl. 
Years later, she finds out that this was not so.  "After Ruth May you were my youngest, Adah.  When push comes to shove, a mother takes care of her children from the bottom up." 
Other instances of betrayal in The Poisonwood Bible:
Nate Price betrays his family in many ways.  He is a Pharisaical, abusive husband and father.  But his ultimate betrayal comes when he insists that the family stay in the Congo when independence, with its almost certain violence, is coming up and the mission is recommending the family evacuate.  His willingness to sacrifice his family ushers in an era of starvation for them and ultimately leads to the death of one family member and the dispersal of the others.  But when Orleanna tells us his back story, we can understand Nate's motivations, even if we cannot forgive him.
One major theme of the book is the betrayal inherent in colonialism.  Belgium ruled the Congo very harshly, then suddenly granted it independence with no training or preparation.  Nate Price, who is ostensibly coming to do God's work among the Congolese people, refuses to learn about their culture, and disrespects, offends and misunderstands them at every turn.  Some of the white characters (Orleanna and Leah) struggle with guilt over their own behavior in the Congo.  Their stories explore the question of whether it is possible, as one of a race that has practiced colonialism, to come to a foreign country without participating in betrayal.  As Orleanna puts it in her introduction, "We stepped down there on a place we believed unformed ... But what else could we have thought?  Only that it began and ended with us.  What do we know, even now?"
The twins, Leah and Adah, have their own issues with betrayal.  Adah was born with a slight brain injury that caused her to limp.  Both of the twins believe that Leah, who was born healthy and strong, somehow caused this in the womb by taking more than her fair share of nutrients or space.  Thus Adah resents Leah for being the healthy, strong twin, and Leah feels guilty and frustrated with her sister.  Both of them deal with this on their own separate tracks, and it is not until middle adulthood that they become friends. 
Orleanna feels guilty for the fact that Ruth May dies.  She wishes she had stood up to Nate before such a thing could happen, and gotten herself and her girls out of there.  She feels that she betrayed them, when in fact, she was doing the best she could.  In fact, for most of the book, the older girls spend a lot of time blaming Orleanna for not being able to make things better: She cannot make a decent meal out of the food that's available, she goes all passive and silent when Nate goes on an abusive rant ... she cannot save them from their father or from Africa.  It's not until the end of the book that they realize she did a very good job with them.