There's a lot of death going on in this book, from the title—Dead End in Norvelt, duh!—to dead deer, to tales of historical slaughter, to the murders taking place in the town. But there's no consensus on how to feel about all that death: it's seen as scary, peaceful, and (sometimes) necessary. And, of course, it also allows important aspects of people's lives to be remembered and celebrated by the community. A healthy fear of death is good for the survival of the species, but Jack's squeamishness goes way beyond healthy. When he's able to stroke the dead deer respectfully at the end of the book, we know that he's made major progress in growing up—even though he still covers up the reflection of himself that he sees in the animal's eye. Hey, we said "healthy" fear, not "zero" fear.
Sometimes, death is a mercy. Although Mr. Spizz poisoned the old ladies, most of them were in horrible pain from illness or injuries.
Gantos's murder-mystery plot is less important to the book's overall message than Jack's maturation.