Spike acts atypically for a vampire on a number of occasions:
- When he "turns" his mother, he seems to believe he's doing something good for her, as opposed to something to torment her or others.
- While perfectly willing to fight and kill, he doesn't seem driven in the way that Angelus, Darla, the Master, or even Drusilla were.
- He seems to have a genuine fondness for Dawn (over and above his attraction to Buffy).
- He chooses to resoul himself at the end of season 6 - something one would expect the demon in control to not like the thought of (comparing Angel/Angelus).
Similarly, Harmony seems more like someone who knows how she's supposed to act as a vampire, but isn't all that good at it or devoted to it. (Note - haven't read the comic series seasons 8+ yet; I know she's involved there, and that might change my opinion).
In her case, as a human she seemed most comfortable having others around, and basically took her own identity from them. This would tend to explain her behavior as a vampire as well.
However, most vampires do seem to retain the memories of their human lives, but that all seems subsumed to the demon that replaces their soul (I think that's how it works, though I could be mistaken).
for both Harmony and Spike, much more of their behavior seems similar to their human behavior than the other main vampires we know.
And, in neither case does it seem likely that it's due to the immense strength of character they had a humans.
So - Are some of the demons that take root in vampiric transformation process less strong than others? Or is there a better (in-universe or at least canonical (creator comments outside of shows)) explanation? Or, for that matter, have I horribly misunderstood how the conversion to a vampire happens, with no other demonic spirit/consciousness/whatever being involved?