I think that's a question for people who profile serial rapists/murderers and study victims of rape/torture. But we can guess.
I've seen a few crime shows in my day, and according to them, it's possible to break a person the way Angelus did Dru. Soul or no soul, it's not uncommon that victims turn into predators, and that people who are broken grow attached to the people who hurt them. The victims learn to love the people who hurt them. And for Drusilla, a person who even as a vampire wants family, Angelus was the only thing left for her. My guess is that she now believes that Angelus is the only family she can have, and that he hurts her because he cares. Or maybe she knows he truly doesn't give a damn, but she wants him anyway. Maybe she's addicted to the abuse by now, maybe she needs him to hurt her.
In What's My Line, when she's torturing Angel, I always wondered if she faked her pain over her dead family. I think that she still hates Angelus for what he did to her family, but she can't ever become someone we could call "healthy".
So she boinks Angelus because she's a vampire with needs, and because he's family.
Maybe she even thinks she belongs to him, because she's filthy?When she was alive, she was a outcast, tortured by her visions. Maybe she still thinksshe deserves to be punished, and that's another reason she wants to be with him? Even tough she's evil, that doesn't mean she still thinks she's worth something?
She's insane, so who knows how she works?