In every neighborhood, there exists an unspoken belief that danger comes from far away—from dark alleys, unfamiliar cities, or strangers who look suspicious. We lock our doors at night not because we fear those we know, but because we fear those we don't. Yet history, psychology, and countless real-life cases reveal a far more unsettling truth: sometimes the greatest threat is not a stranger at all, but someone living quietly next door. The idea of "The Killer Next Door" challenges our deepest assumptions about safety, trust, and human nature.