Amazon did that by offering superior prices, customer service, product range, delivery, etc. That's only a problem if Amazon raises prices or cuts quality after it's competitors are vanquished. What we've observed is just the opposite. Amazon continue to relentlessly push lower prices, better customer experience and faster delivery.
Competition is not a goal in and of itself. It's merely a means to the end of improving consumer welfare. Antitrust law is careful about this distinction. It's not illegal to be a monopoly because you delight you consistently delight your customers much than your competitors. In fact just the opposite, restraining a superior product from growing its marketshare would be actively harmful to consumer welfare.
