Not sure what you guys are on about... I've seen worse stuff than this rated 8 - 8.5 / 10. I think a lot of people are being affected too strongly by the title.
For the "boring" comments... go read Bastard or something. You're expecting an action manga and got an investigation instead.
For slavery... MC refused to buy even when the shop keeper begged him to and only gave in when he saw the girl was in immediate danger of abuse or death. Sure, he wasn't immediately condemning all of society for allowing slavery at that point, but that's largely because he already viewed society as entirely rotten. Remember, the first thing that happened when he was summoned was basically a slave auction.
I doubt the Japanese have the same attitude in general toward slavery as the typical American anyway. In the US, condemnation of slavery is a social requirement (like supporting feminism, condemning racism, or condemning ephebophilia), because of the history of black slavery there and the various movements to abolish it. The Japanese don't have that same history.