Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Detached, Cynical
One of the reasons why reading this novel can be disturbing is that Oskar can be a very difficult character to sympathize with. After all, the guy talks about the deaths of those closest to him without much emotion, and he observes horrific events with utter detachment. For example, while people are being raped and murdered by Russian soldiers in his basement, he occupies himself with watching ants and picking lice from a soldier's hair. When he sees the soldiers hanged from trees, he pays more attention to the quality of the handwriting on the signs around their necks.
Oskar has an insane superiority complex. He feels he's a genius trapped in a world full of common people. So this tone of condescension permeates the novel. Even when people visit him in the asylum (not a fun way to spend an afternoon), he remarks,
Once my visitors […] have managed to convince me, by their tireless attempts to rescue me, of the high quality of their brotherly love, they find renewed joy in their own existence and depart. Then my keeper arrives to air out the room […]. (1.6)
It doesn't get more cynical and detached than this. Remind us never to visit this guy.