It is a little difficult to answer your question because you mention Night by Elie Wiesel but have omitted the second book for the synthesis essay. However, both books concern the Holocaust, so there are themes that can apply to both. Moreover, you mention as a possible theme that unless people experience a situation themselves, they seem to underestimate how much a dire situation can drive human behavior. This would make an insightful theme.
You specifically cite the scene in which Elie feels guilt because the overwhelming instinct to survive makes him reluctant to give the remainder of his soup to his father. He likens himself other prisoners who he believes have lost their humanity under these horrible circumstances. However, it is very important to keep in mind that Elie does give his father the soup, even if it “was with a heavy heart.” Wiesel writes,
No better than Rabbi Eliahou’s son had I withstood the test.
However, Elie is being too hard on himself because of the guilt he feels. In reality, he fared much better than Rabbi Eliahou’s son. Another thing to keep in mind is that, according to The Wiener Holocaust Library, the food rations that concentration camp prisoners received came to 1,300 calories a day, which is little more than half the 2,500 calories per day recommended for men. This meant that prisoners were starving, which makes Elie’s sacrifice even more significant. He rose above the instinct to survive to help his father.
You also mention that we are shown incidents in the book in which Jewish prisoners inflict pain and suffering on fellow Jews as a means to survive. This is true, and we see it in the behavior of some kapos and other Jews who have been given some power. However, there are also examples of prisoners who behave with humanity towards their fellow prisoners. For instance, one of the first men that Elie and his father meet upon arriving at Auschwitz instructs them both to lie about their ages in order to survive. Also, one of the kapos is kind. Elie writes,
The Oberkapo of the fifty-second cable unit was a Dutchman, a giant, well over six feet. Seven hundred prisoners worked under his orders, and they all loved him like a brother. No one had ever received a blow at his hands, nor an insult from his lips.
Later another refuses to hang “the pipel.” Elie says,
This time the Lagerkapo refused to act as executioner. Three SS replaced him.
Based on these instances, it would seem that a powerful theme could be that although dire situations often drive people to do horrible things in order to survive, there are still examples in which people retain their humanity and help one another, even if it means potential danger to themselves. If you choose this theme, three points to incorporate could include the instances above or perhaps the example of the girl who helps Elie after he has been beaten and tells him to wait until he is in a better position to get revenge.