There is one reason to buy your own company's stock and a dozen reasons to sell it. Executives are paid largely in equity, so selling is how they turn compensation into a diversified portfolio, a house deposit or a tax payment. A large sale is therefore weak evidence about the price on its own.
What is worth attention is the shape rather than the size: a sale that breaks an insider's own pattern, several insiders selling in the same short window, or selling into strength shortly before a known catalyst. Sales made under a Rule 10b5-1 plan were scheduled months earlier and carry almost no timing information at all.