Psychopaths are, broadly speaking, unable to feel empathy, fear or emotions. They feel no compassion and have no problem with tricking, manipulating, harming or stepping on others to get their own way.
They have fascinated us ever since we figured out what they were back in the 19th century. Back then, doctors noticed that some of their patients were exhibiting all of the behaviours of "insane" people, but without the insanity part. They called it manie sans dlire, which roughly translates as "insanity without confusion" as those patients tended to appear to be completely compos mentis, and yet still engaged in risky, anti-social and self-defeating behaviour.
We tend to picture psychopaths as dangerous, cold, calculating killers but the reality is a bit more complicated than that.
Sure, most psychopaths aren't exactly cuddly, but we should think of psychopathy as less of an on/off deal and more of a broad spectrum consisting a various disorders. As such, the world of the psychopath is mysterious, complicated and often surprising.
Because psychopathy is a complex and mysterious condition that we still don't fully understand, psychopaths can sometimes be tricky to define. Not all psychopaths want to wear your skin or stab you in the shower, they crop up in every walk of life in all shapes and sizes and you can't necessarily tell by looking.
It could be your boss, it could be you mum, it could even be you.