A lot of things suddenly get explained when one becomes aware of the traits associated with Psychopathy and Sociopathy and how around 5% of humans are high in the spectrum for one or the other.

In my own experience, certain kinds of behaviors are incredibly hard to believe in (deep down, emotionally, even if intellectually one does) when a person has an average or above average level of empathy: when you have a normal level of empathy it's pretty hard to put yourself in the shoes of and naturally accept that some people are casually and without any feeling of guilt the kind of person who, for example, couldn't care less if their personal-upside-maximizing actions hurt even little children or puppies as long as they didn't get reprisals from others for doing it.