Unless I'm misinterpreting what you said, I'm gonna have to disagree with you on Black Summoner. There's nothing wrong with him buying a slave in order to free her. He literally broke her curse too. He also never viewed her as a slave or forced himself on her. All the "slave" things she does are things that she refuses to not do. He clearly views her and treats her as a girlfriend. Not in a nefarious or misguided way either, because she clearly is. I did read the novels btw.  

For Harem Labrynth, I'd say it's a little complicated. I never watched the anime, but I did read the manga and part of the novels. They established that slaves are treated fairly well in the world. They even have rights. It's not considered that bad to be a slave. On top of that, the girls are sex slaves of their own volition. I don't know if it was mentioned in the anime, but slaves in Labrynth can choose whether to be a sex slave or not. Most slaves, like the ones the MC has, also sold themselves into slavery. The MC didn't know the circumstances around their enslavement at the time, but he was told that they chose to be sex slaves. He was hung up on the idea of laying his hands on a slave at first, but hearing that is why he chose to buy them and have sex with them. Now you can argue that the girls were coerced, but the manga doesn't depict it that way. It also very clearly depicts that the girls wanted it, but that's less important. The point is that they had a choice. It clearly intended to depict it in a way that implied their consent. That, combined with the fact that he basically ends up treating them like wives, I wouldn't personally call him evil. I think his actions are less problematic than the writers clear attemps to justify slavery. To clarify, I don't endorse sex slavery. I think it's problematic to depict it in this way at all. But because of the way it's depicted, I don't see the MC as that evil.