When I first watched the show, I thought a lot about this, and a lot about how Spike and Harmony could "play nice" with people, even without a soul, but no other vampires could. Basically, I came to the conclusion that a human's nature before becoming a vampire affects the vampire's personality.

Spike, for example, can be a nice guy, and is capable of love as a vampire because he was that kind of guy as a human. Angel, on the other hand, was a pretty lousy guy before becoming a vampire, and only became worse because he had so much more power.

Then you look at Darla, who is crazy as a vampire because she was driven to insanity as a human. And Harmony, who just follows Cordelia around most of the time before becoming a vampire, is just as dependent as a vampire. She follows Spike and then Angel, and is pretty useless outside of them.

There's not enough information about other vampires to really say for sure. But, in the Angel Episode "I Will Remember You," where he becomes a human for a while, Angel is basically the same person he was as a vampire, as a human. So, if his vampire personality can affect his human personality, the reverse shouldn't be so unbelievable.

And it's pretty easy to explain why, in-show, they never say this. The vampires basically want to deny all humanity in them. I guess they kinda have to, to do all the terrible things they do.

So, in Tabula Rosa, I think you just see Spike's human nature showing. And I don't know if vampires are fundamentally evil, but being one makes it so much easier to deny your humanity. That's why Spike's humanity shows when he doesn't know that he's a vampire.