Strangely enough , as much as I love the description above, it would appear that dark side spirits exist only as a manifestation of the dark side of the force, the devs have been quoted as saying that in cannon only the light side had a path to immortality, ala force ghosts dark side spirits were not manifestations of a previous dark side users spirit but current manifestations of the dark side of the force as the force user perceives it, and not long gone spirits of dark side force users.
The Clone Wars and less so, Rebels appear to bear this out. Max Brooks was quoted as saying all the dark side leads to is self perpetuation of hate rather than longevity.
Please don't take this wrong, but I might say I hate this. It always made more sense to me to have Dark Side Spirits be more the norm, as they are so individualistic, and fear being forgotten, after their meteoric rise to power, while the Jedi are without feelings, or hubris, by comparison, and they wouldn't see fading into non-existence, and becoming one with the living Force bad. In fact, when Obi-Wan did it, it sort of was a bad thing; he was resisting the path of the Jedi, and placing himself before peace, but in order to prepare the last Jedi for his trial. Later, he even has to tell Luke that he won't be able to do this, anymore, and they say good-bye, in a dream. Yes, I know that this is EU, which they torched, but it made so much more sense. The Sith need to rule everything, even life, death, and time, makes them spirits, sometimes, while the Jedi are just here for the moment, and prepared to live as they are supposed to, die as they are supposed to, and become one with the Force. This is why Yoda tells Anakin not to fear death, because it's normal, while beings like Exar Kun, and Palpatine, clung to existence, as shadows, in order to try and continue their evil plans.
I see it as Light Ghosts are being of cold thought, whereas Dark ghosts are bonfires of emotion. Obiwan never so much as raised his voice after becoming a ghost, whereas Sheeve Palpatine left a psychic stain over half of Endor