I am a fan of police tales, especially crimes that happen on the ground. Each time, I became more sure that human psychology is very complex and cannot be classified under any method, and that reality is always much stronger than imagination and fictions.
The examples mentioned by the author of the topic may represent the most extreme cases, but there are much worse cases, especially cases of free killing by serial killers and sociopaths.
The idea may sound terrifying. But we should not really trust anyone completely.
Same here, I love to watch detective movies with suspense and listen to real life stories about murderers who got caught.
It's always shocking when a parent kills a child or a child kills a parent. There bond there should be above everything else. I'd never hurt my child, I'd rather take the hit myself. People who murder their own children must have lost basic instincts that are coded into our DNA.
To answer the OP's question, I trust my wife because we've been together at times when we both had nothing and we've managed to support each other. I hope when my daughter grows up I'll be able to trust her too.