I'll keep this brief without any spoiler and assuming you know what you are getting into because you have heard about this before coming here, and you know this is a hotly debated topic right now with the anime coming out.

Recently a bunch of manga has been getting made with the idea of "MC was a good person until someone really forced him not to be one." This is not supposed to be a happy story. MC doesn't have the capacity to forgive everyone like Jesus Christ. He went over the edge as a lot of people would if they were in his shoes. The only person finding pleasure in this is the MC themselves. These stories are for the self-satisfaction that not everyone has to be the stellar individual who puts up with way more shit than they needed to in the first place. I could describe what happened to the MC with tags you find on disturbing hentai and it would only make more sense.

Most anime pander to the "It's my duty," and all the jazz. This one decides to allow the coin to fall on the other side for once. Granted, these people he is going after are not good people. It simply shows dirty work done by someone who is down to get dirty.

This is also pretty bad on its own, this is a story that will never be socially acceptable, so don't get into it and then complain about how graphic it is. It isn't for kids. Recently anime/manga has been developing in a way where that rating tag is actually being used to the fullest. (ex Interspecies Reviewers) Not all Anime/Manga is for kids, and just because it isn't for kids doesn't mean it is hentai. If you come into this expecting a fap... I'll be pretty disappointed. That isn't the point behind this kind of story.

Why is this review relevant to where I'm posting it? You need to be aware of what you are getting into and make a choice after knowing what you are getting into. Don't watch this after seeing Fairy Tail and give it a 1 because it wasn't as "friendly" or whatever you wish to call it.