From the beginning, it already foretold that it was a tragedy but later when Kaneki grew strong, we all had a false hope that things would be changed, Kaneki would protect the weak, beat the evil and together with Kōtarō Amon, the young official of CCG (Commissioner of Counter Ghoul), human beings and ghouls may someday find a way to mutually exist in the world. But the world was simply wrong (that's the central theme of the story!!). We conflict with each other for existence, no matter it was between ghoul and human, between ghouls or even just between human. How we crave for existence and how miserable in such craving ("I must eat, to eat is to plunder, to protect, to lose, to make mistake").
I love the way it presents the story, which does not draw everything out, not depict the plot explicitly. The work is quiet poetic - the constant counting of numbers (not just to remain awareness of the pain of existence, but also means the counting of the minutes/seconds of your life), the encounter of the good and the bad Kaneki (the realization of the futility of all your hard struggles) and the final vision of the smile of clown.
Oh yes, clowns. The mocking of clowns. I don't understand why the readers turned hating Uta at the end. He may be unscrupulous but the tragedy of Kaneki is to a very large extend due to his own personality. The extreme Type 2 Enneagram - to validate his own worth by the extreme self-sacrifice, just like what Toka said, it is a passive manipulation to others. The unsympathetic words of Uta remind me of Macbeth's famous words:
"And all our yesterdays have lighted fools