You don't notice someone has no empathy until you see them behaving horribly. The word is being used technically to refer to a non-empathic person, but people assume that all non-empaths behave horribly because (with rare exceptions like this neuroscientist) all the visible ones do.
Conversely, I used to assume that having empathy implied treating others well - that all people who were especially empathetic also wanted to be nice to people.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.