I am not a trader but I believe stock markets take place on an order book with bids at low prices and asks at high prices, with the market clearing in the middle. The upshot of this is that your trade for a million shares will sit on the books like everyone else's, waiting for a seller. If somehow a seller entered a market order to sell 1 million shares for whatever price it can get, and somehow the computer matched it to your order, then that would actually be the last price traded, i.e. a crash.

tl;dr: your strategy is more likely to contribute to a stock crashing than you think.