Think of Mr. Spizz as a more sinister version of Henry from Eureka: he's a jack-of-all-trades, a man-about-town, and (apparently) a pillar of the community: as a member of the Norvelt Association for the Public Good, a volunteer firefighter, and a volunteer policeman, he's got his eye on the whole town.
Literally. He's also the "town busybody" (he always seems to know what's going on with everyone) (2.15), and he seems to think that the whole town should operate just the way he wants it to—even if that means he's got to murder a few old ladies along the way.
We get a heads-up from early on that something isn't quite right with this guy because of the unflattering way he's described: he has an obnoxious, fog-horn voice (e.g. 26.59); he's a "heavy breather" (3.1); and he has gross habits, like picking yellow earwax out of his ear (15.25). As if that weren't enough, he also rides around town on a ridiculous "giant adult-sized tricycle" (3.17).
This tricycle clues us into the fact that Mr. Spizz, like Mr. Gantos, is nothing but a really old man-child. When he doesn't get what he wants, he obsesses about it for, oh, almost half a century—and then murders people to get his way. And you know what? Just like Jack was in danger of growing up to be just like his dad, he was also in danger of growing up (or "growing up") to be just like Mr. Spizz.
But thanks to the guidance of Miss Volker and one important summer, Jack escapes that fate: he actually learns to grow up, be a man, and take responsibility for his actions—while not losing the kindness and compassion that makes him want to help out others. So, maybe we should thank Mr. Spizz after all—for showing Jack how not to be.