They're not complete psychopaths as they do have emotions tied to morals. However, a surgeon can lose a patient on the operating table and feel little to no empathy. A CEO can feel no empathy throwing out 10,000 employees to save on the bottom line and his/her bonus without batting an eye. Some soldiers on the battlefield can take lives without suffering any sort of PTSD or remorse. That takes a certain amount of psychopathy.
One can acquire psychopathic traits through desensitization.
I'm starting to wonder if we're having two different conversations here. I didn't claim Dexter is a great guy to hang with or that he's a model of society. He is what he is, a serial killer with an uncontrollable urge to kill. That doesn't make the show and its stories any less brilliant.
Still trying to figure out what you're getting at here. There are varying levels of psychopathy and not all of them are demented (or relating to dementia).
Psychopathy is more complicated than simply a mental illness.
http://www.psychologytoday.com/blog/mindmelding/201301/what-is-psychopath-0
You'll notice these traits are very similar to what one would expect from special forces or secret service. Some traits can be acquired but some of these jobs have very rigorous and extensive psychological profiles performed on their applicants to determine predisposition.
What position am I defending? I said I like the show and that's it. I'm not trying to convince you to watch it based on some delusion I have. Now we've drifted into the subject of psychopathy and what defines a psychopath.