What would make parents lose their instinct to protect their own children?
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).