There does to me seem to be a significant difference between neighboring communities enslaving each other (and knowing that they could be next) and transporting people across the world to suffer and die under a regime that denied them any humanity whatsoever.
I see. So despite the sources saying that "these subject peoples were then exploited for cheap labor", and, presumably, were also used for the documented human sacrifice, we know that they otherwise treated their slaves relatively well? How do we know this?
You also say "enslaving each other", but from what was written so far it seems to have been mostly Buganda enslaving its neighbors - they didn't take turns.