After reading the whole manga (in Russian) I can say that it was much, much better than I expected. Brutal mangas have a tendency to get dumber and more meaningless as the story goes, as well as the story in them just being dragged on for forever. Nothing of this happens in "Massacer Happy End". The story logically leads to the conclusion, you can clearly see the planning behind it, and it still only has about 50 chapters.
Other than this, the concept overall is intriguing and is never forgotten about, no other mystical powers or such come in play, it is pretty straightforward.
The cast has a whole lot of characters, some of them are really charismatic. My only concern is that they are pretty two-dimensional, which is okay, because the focus of the story is the main character, but it is still a shame. In contrast, the main character and his motivation, as well as his mental state are shown very well, really makes you feel sorry for him, while making you question his methods.
As I said before, the story is written very well, every new element leads to a logical conclusion, while not dragging the attention away from the main plot. There is some number of twists that keep the story interesting. Most of these twists are very good, some of them are questionable, but not completely outrageous.
I'd give this manga an 8/10, not a total masterpiece, it has some clichés, but it's still a really, really good read. I hope it gets translated to English at some point.