I think Drusilla hates herself. She was so good, a tool of the powers and a pure young woman and then to become what she is, evil and vampiric, finding pleasure in torture and killing of the worst kind, I think deep down under all of that insanity she hates herself. And I think that’s why she prefers Angelus to Spike. Spike loves her, loves who she is, the thing she has become, is utterly devoted to her, Angelus could care less about her, just uses her. So she goes to Angelus because she hates who she is and can’t stand the fact that Spike loves what she has become.