I am one of those people. It comes from my mother being a union leader, my dad being a complainer with a short temper, and being unpopular in school, teaching me to not care what others think of me. It doesn't bother me one bit to stick my neck out at the workplace and (productively) call out BS, or to make some Comcast customer service representative's life hell until they rescind some BS charge on a friend's account. I wouldn't call it "assertiveness" - I don't consider myself very assertive - but more a very low tolerance for BS and "injustice".
This means that I've spent most of my adult life learning not to sweat the small stuff (especially since moving to a major city), but rather focus my energy where either it will actually benefit others, or against someone who is actively taking advantage of me in a harmful way. I have to consciously remind myself that to do otherwise - to confront every line-cutter and litter-dropper - will at best drain me of energy for fights that matter, and at worst, invite trouble I'm not able to handle.
Relevant to the article, I credit this same ability with having helped me escape two emotionally abusive, back-to-back, relationships (both made possible thanks to my otherwise lack of assertiveness in social areas).
I don't think I'd call myself a psychopath, though I share some traits (e.g. low - but not absent - empathy). If my parents hadn't instilled in me a strong sense of morality maybe I would be.