I mean, yeah you could control Garithos for a long while ideally, but it’s worth noting even with said Dreadlord one of the Ogres from WC3 DOES break out of his control in Vanilla. Don’t ask me how you break Banshee possession, I don’t know, maybe the Banshee just vanishes. You might be able to setup an unfortunate accident, but I would think the humans would probably get pretty suspicious if Garithos tried to take a bath in a river with his armor on and just drowned.
As for actions, yeah the Forsaken absolutely do horrible stuff to their enemies, but it’s not like humans, especially humans in the Scarlet Crusade, have a compunction with killing Forsaken even when they are surrendering(Something even the Alliance has done.) or torturing their enemies to death. Because by the same token that Frank Castle can be understandable and sympathetic despite doing horrible things, a big component of that is that his enemies are also just plain awful people, and humanity has not been doing that good a job of showing the Forsaken they deserve trust, leading to these actions because this happens even if the Forsaken in question has done nothing to humans and the humans in question don’t even know what they are.
The self fulfilling prophecy goes both ways.
As for Akiyass: Yes the Jewish people were not powerless in Germany but the fear that the Jews would somehow hurt the German people was unfounded, there was no malicious intent as far as I could tell. There is a vast gap between economic control over key points in a nation making people somewhat uneasy vs the majority of that population waving a gun in your face. The threat is much more immediate, understandable, and more importantly then that there’s consistent proof that humanity will just attack you for being undead. It’s not an isolated incident between the two groups.
A little addendum to Lordaenian History: The camps were made to try and save the orcs, not to enslave them, they would make terrible slaves anyway due to the lethargy rendering them immobile. Slavery, by and large, is the legacy of Blackmoore specifically who took over after Danath could no longer serve as wardens for the camps. Human experimentation and Abominations are definitely a thing that the Forsaken do that is morally reprehensible, but not every Forsaken is a member of the RAS either and judging by captain Hyde choosing to respect the spirits of the dead, and the Desolate Council sympathizing with humanity in BtS, they don’t seem universally inclined to believe either of those things are good.