I think people who give it 1 star just don't have the stomach for dark fiction. All anybody complains about is how there's content that's hard to watch. While I can understand it would be difficult to enjoy watching people be killed and raped, that's what makes this dramatic. If you're too weak to simply acknowledge the scenes are for drama and atmosphere... don't watch it.
Overall I think many can agree this was a solid anime. Plot seems repetitive but this anime still makes it interesting to watch. I think that's impressive. Main character isn't overpowered at all and mostly uses his companions abilities and his intelligence to win anything. While there are slight harem elements they are very subtle and rare. Action was good. Main character was likable in a batman sort of way. Side character chemistry was excellent. This is an easy 5 stars and a great anime to watch.
Although if you are a sensitive baby, I do not recommend. You might puke