What would make parents lose their instinct to protect their own children? I've seen footage of the holocaust where parents and their children were shot, with the parents just standing there, doing nothing to protect their children.

I have enormous difficulty in understanding the mindset/attitude/reason for this, fatalistic?, conduct. Being a parent myself, I know I would just go berserk.

What would make parents act like those in the footage?
Sounds like you have had a distressing encounter with "learned helplessness" as madsun also described. Learned helplessness is a well-studied psychological phenomenon and reaction that explains the terrible situation that you have described. It is situational and explains the "fatalism" that is so difficult to understand. The simplest explanation I can find seems to be 


  a phenomenon observed in both humans and other animals when they have been conditioned to expect pain, suffering, or discomfort without a way to escape it (Ackerman blog, 2019)


Chris describes the different types of learned helplessness in an another question 


How does one escape learned helplessness?


References: 

Learned Helplessness: Seligman’s Theory of Depression (+ Cure). PositivePsychology.com blog (2018, March 24).