The central mystery of The Westing Game, really, ends up being about discovering who Sam Westing is. In fact, there's $200 million riding on finding Westing's fourth identity. For most of the book's characters, while they think they're competing in the Westing game, what they're really doing is establishing the identities they should have had all along. Through the process of the game, they're all redefining themselves. Maybe the message here is that there's more value in figuring out who people are – and who they want to be – than worrying about imaginary murders and crimes that never took place. The game helps the characters to discover and reveal their true selves.
The central mystery of The Westing Game isn't who killed Sam Westing, it's the question of who any one of us really is.
Although The Westing Game turns the murder mystery trope of finding the criminal's true identity upside down, it preserves the traditional idea of positioning a person as the answer to life-altering questions.