True. The fact that the dunmer were raging racists didn't stop them from solely enslaving the beast races, but they did show a strong preference for Argonian and Kajhiit slaves. Its not like they went raiding dumner settlements outside Morrowind for slaves....
True, but that probably has other reasons. Becuase those towns didn't exist, and all other towns near them were either in Cyrodiil or Skyrim. Both provinces with enough influeince in the Empire to declare outright war and seriously hurt the Dunmer, which wasn't good at the time because they were also fighting Dagoth Ur.
If it had been limited (as specified by the treaty with the Imperium) to internal maintenance of a system slavery, maybe not. But even after that, there was no abatement in cross-border raiding and capture & enslavement of Black Marsh citizens. So I say hell yes, smash those bastards. Finish what Red Mountain started. They are so proud of their history, let them live like their stone-age ancestors. (Actually, the Ashlanders pretty much do, and aren't such bad folks).
There is certainly an argument to be made for that, but like I said slavery was already abolished. This was therefore no longer a fight against slavery but a fight for justice/revenge. Whichever you prefer, and then it becomes a matter of complete personal opinion.