My ever deception in humananity-"Their Darkest Hours"

I am reading a book on how people were tested in extreme environment, situations such as in war. Someone is nice in a normal environment can be pushed by external influences and forces to be real bad in order to survive. That's why there were many Jewish killed, many chinese women raped and one was eaten (yes, this is true, and only one story was told, imagine!) by Japanese soldiers...But what was worse that there were many betrayals happened in these extreme circumtances, friends sold friends, and nobody can trust nobody, let's not mention about asking for help.

We have tried to do better in some ways, I suppose to not let these horrible things happen again. However, it is in our nature that we are bad, and only under certain circumtances, we can really know ourselves and others. Certainly there are always decent human beings who despite whatever situations they are in, they will do the right things. If not, we wouldn't have any survivors in WWII. We indeed do not need religion to make war but hatred, selfishness, greed and the belief in superior race.

How can you trust if many times you have been betrayed? How will we behave in our darkest hours?