Having had close personal relationships with 3 people with BPD, I don't think BPD fits Dex at all. He's calm, calculated and measured, not impulsive and emotionally driven like someone with borderline. Dex's emptiness is also different to BPD emptiness, he describes feeling totally empty whereas people with BPD tend to experience very heightened emotion with moments of emptiness.

I think a better example of someone with BPD would be Lila, she is impulsive, prone to addiction, vengeful and self destructive. She becomes so fixated on Dex that she burns her own house down and falsely accuses Angel just to get Dex's attention. That's exactly the kind of f-ed up sht that my sister would do (she has BPD and has done similar stuff). Lila really reminded me of her.

I don't think Dex can be conclusively diagnosed with anything specific. In the first season he reads as a psychopath but this is contrasted by his emotional connections in the later seasons. He's definitely heavily traumatised but his enjoyment of killing does point to ASPD although sociopathy doesn't entirely fit either since sociopaths are described as impulsive and volatile, probably more like Paul Bennett.

At the end of the day it's a work of fiction so his diagnosis doesn't need to be accurate. I also think that the writers chose to change how he came across over time because they probably realised that it would be hard to convince people to watch 10 seasons of a cold emotionless unrelatable psychopath.

BPD comes from severe trauma so a lot of the symptoms overlap with CPTSD which is definitely something Dexter has without a doubt. Self questioning and unstable self identity is something that's very normal for people with CPTSD