Were characters in The Giver (other than Jonas and the Giver) believable?

The characters’ choices in The Giver should seem unbelievable and horrific to its audience.
For example, Jonas’s father, a seemingly “good” man, does not show any guilt after euthanizing the smaller twin in Chapter 19. While watching this scene, Jonas feels horror and repulsion, and he cannot fathom his father’s willingness to casually commit murder to save his community from the small inconvenience of interacting with two similar-looking people. The Giver’s audience should feel the same disgust and disbelief, especially since in our society, Jonas’s father would be seen as a cold-blooded murderer who would receive a harsh penalty, perhaps even capital punishment, from our legal system.
Similarly, Fiona, a character admired by Jonas for her gentleness and good nature, uses a discipline wand against the elderly. Our society would also see this as an “unbelievable”, inhumane crime. We would use the term “elder abuse” to describe her actions. Therefore, from one perspective, the characters in The Giver are not believable.
However, there’s another perspective. One could say that the characters in The Giver make believable choices given the fact they are brainwashed into submission by the constant rules and regulations of their society. Even if we look back only one hundred years into our history, we see that when unjust rulers seize control of countries and begin extensive programs of propaganda, repression, and intimidation, seemingly moral people commit unbelievably horrific acts.
For example, you may have studied genocides, the deliberate killings of large groups of people, in your Social Studies or History classes. I have included a reference link to endgenocide.org to show that genocides are horrifically common and that despite humanity’s best efforts, they continue to occur. I have also included a link that discusses author Lois Lowry’s feelings about the similar timing of The Giver’s publication and the worst days of the Rwandan genocide.
Therefore, given that our history shows that ordinary people are capable of terrible crimes, the characters’ inhumane actions in The Giver may not seem so unbelievable after all.