Since there is already a really interesting review that tries to analyze this manga in a deeper way I don't see the point in doing the same, so I will speak more about what I think about it instead of studying it.

For me the first 125 chapters (or so) of Monster are close to perfection, the narrative and storytelling is probably at the peak of manga, everything told is really interesting, extremelly well interlaced, every character, plot and subplots advance creating a fascinating world and narrative.
About this, one thing that I liked is how it portrays the police, the academical world, the love relationships, the layers of every character in a frank way without the usual idealizations, and showing the dark side that everything really has.

After the first 100 chapters when we already have enjoyed a great demonstration of storytelling Monster achieves to step up with all the part about Bonaparta's books and how they relate with the story of the twins and the orphanage. That's when everything enters in a more symbolic point, with a deeper meaning and also relations with psychology, conspiracy aspects of it and so.

The problem is how everything comes to a resolution, the presentation and developement of this story is much more interesting than the way it resolves. The final act of Monster even if it's good if you don't compare it to what you previously know if you do it's not at the level of the previous. I think it would be really difficult to make all the pieces that were presented fit for a satisfying end, and I don't necesarilly talk about a traditional ending where everything is explained and so, I also like open endings, abstract ones, etc. My problem is that Monter ends in a kind of lazy way, like a regular thriller (and Monster already became more than that) where when things come to an end everything explodes, gun fights, killings, tragedy all justified by that idea of the perfect suicide of Johan but that minimices the deeper meaning that everything they presented to us before could become.

So many great moments while reading what was before the end, so many revelations, so many pauses thinking about what every little thing could now mean, but that final part at the little town was just like reading it with an authomatic pilot, not so many things to think about, to feel, to discover...

In short, a fundamental manga that needs to be read by everyone, great story, mood and characters (if i'm sincere the twins and Tenma are the ones that I felt less interested with, even if they are what really drives the story).