How come people with gold teeth get a bad rap in old movies? What's wrong with losing a tooth, exactly?
At any rate, at the beginning of the movie, Kevin's startled by Harry's twinkling gold tooth—even though Harry is pretending to be a good guy, a cop. Later on, when Harry and Marv are driving around in their fake plumbing van, looking like a couple of burglars, they almost hit Kevin.
When Harry smiles at him, Kevin sees the tooth twinkle again and somehow he knows—this dude is suspicious, and he was probably sneaking around the house, casing the joint. Later, the gold tooth gets knocked out of Harry's face by one of the booby traps Kevin hits him with (maybe it was the paint can).
Finally, at the end of the movie, after Kevin's cleansed Marv and Harry in the fires of pure pain and then got them arrested, Kevin's dad Pete finds the gold tooth lying on the floor of the house.
He says aloud to Kevin's mom, "Honey, what's this?" It's the one clue left in the house that Kevin took on a pair of burglars, a fact Kevin hides from his family.
And why does Kevin hide that fact? Is it because he doesn't want them to realize his massive capacity for bloodlust—the beast that ranges within? Maybe. Perhaps he wants to keep that beast on a chain.