I think AR’s comment could be a little misleading there; she says Gabrielle is “cold and selfish and essentially a bad human being” and then, separately, that Gabrielle is “a bad vampire.“ 

^Reason enough for AR to dislike Gabrielle. Gabrielle defended him against his father and brothers, helped him run away to Paris, stayed with him for a good decade exploring the world, and then wanted to go off on her own. Idk if I agree that she treated him SO badly, though she could have shown kid!Lestat a little more affection, he was starved for it. 

I don’t think AR means that coldness and selfishness = bad vampire. One would think that those are "good” vampire qualities.

(^Catherine Deneuve bc of reasons)

What’s MOST likely is that AR modeled Gabrielle after someone AR is not particularly fond of, thus our dear Gabs was doomed to never be able to bask in the glow of AR’s love ;A;

So who’s an example of a “good” vampire in AR’s eyes? Marius, Lestat, Louis. Vampires who still strive to be part of the fabric of civilization to some degree:

• Marius still finds pleasure in painting, socializing w/ the coven members, he studies history, he’s a people-watcher. 
• Lestat is a sensualist; he wants to crash parties, sample all types of victims, acquire all kinds of toys, be Good at Being Bad, share affection with his coven anyone. 
• Louis does not in fact want to be left completely alone. Even though he’s a huge bookworm, he also has a deep appreciation for art, and does relish frequent *ahem* cuddles with Lestat, bc who doesn’t? 

… Oh right, Gabrielle doesn’t. Gabrielle would rather not be around any vampires or people at all. For legitimate reasons, for sure, but she seems to have little interest in making any connections with civilization, no interest in art or fashion, only a tenuous commitment to her coven,… she wants freedom. Perhaps that’s too lonely and empty an existence for AR to contemplate.

But mostly it’s bc Gabrielle “treated Lestat badly,” and that she doesn’t need or want to be a regular part of Lestat’s family in the way he would love her to be, and since Lestat is AR’s precious bb, Gabrielle’s rejection of Lestat is a rejection of AR herself (which is slightly amusing considering that AR’s commitment to writing Gabrielle in character prevents her from modifying Gabrielle into someone she finds more likable!)(#PROFESSIONAL AUTHOR PROBLEMS)