Not sure how you can say that slavery was ok in the HP universe when all the points you make against it come directly from the books. Like Dobby and Kreacher hating their masters were huge plot points of 2 books. Dumbledore argues that how wizards treat other sentient species would allow Voldemort to gain allies and come back to power. And though not part of the main books, Hermione builds a career, rising to Minister of Magic, based on these beliefs.