In Buffy universe; What makes a vampire what it is?

All BTVS ma cà rồng are supposed to lose their souls in the transformation from a mortal to a vampire and they are all thêm hoặc less vicious monsters, loving to kill and torture an so on.

But what affects how a vampire is like? Angelus's a total, heartless psychopath. Darla seems to have been quite a match for Angelus. YET Spike and Drusilla had clearly a lot of humanity left in them, even though they had the monster in them the same as Angel and Darla. But how come they differ so much and clearly of the other two if they're all supposed to be soulless?

I don't think the script writers knew what they exactly want a soulless being to be like and being this illogic in the matter is pretty much making the losing a soul matter, pointless. Except of course in Angel's case individually it has a great point but generally taken it makes no sense.