I think Angelus "trained" Dru to submit to him sexually. Dru often refers to Angelus and Darla as "Daddy" and "Grandmother (or some variation thereof)" respectively. Dru is often portrayed as being very child like in some aspects, and children tend to want to please their parental figures so Angelus is taking advantage of that by engaging in sex with Dru (plus he also does it to annoy Spike). In season 2, when Spike was wheelchair bound, Angelus was practically rubbing it in his face about what he was doing to Dru and that Spike couldn't do anything to stop it.
The writers needed Spike on the show so they could push forward with Spuffy. However, Spike either mentions Dru or she is seen (in flashback) in every season Spike is in which kind of seems like the writers don't want the audience to forget that Spike and Dru were together. At the very least, in my opinion, Spike hadn't fully let go of Dru because he keeps mentioning her.
There was a way Dru and Spike could've been on the show yet not actively trying to kill the scoobies (I'm using this reasoning for a season 4 AU fan fic I'm in the process of writing) and that is Spike and Dru come back to town for the gem of Amara (Spike's trying to make it up to her for his perceived "betrayal" in season 2) and Dru gets caught by the initiative and chipped. Spike would stick around to protect Dru since she wouldn't be able to protect herself because of the chip but he'd have to keep a relatively low profile so that neither he nor Dru get staked by Buffy. His and Dru's behaviour would still be modified by the chip but in a way that I think explores the relationship more eg is a vampire actually capable of love for another?
.
I think the whole Acathla thing was just Dru following Angelus' lead. As for the Judge, I think Spike and Dru were just looking for a bit of mayhem there. Spike admitted he doesn't want the world destroyed, but also it's not in a vampire's benefit to eradicate all human life, otherwise what are they going to eat.
