The rise of a monopoly is the end of competition and the death of a free and open market. The holder of the the monopoly can then raise prices to a level that extracts an economic rent from the market while simultaneously preventing other competitors from entering the market. This is similar to how OPEC is able to increase production, lower the price of oil, drive fracking and oil sand companies out of business, buy them up, shut them down, artificially create shortages, and then raise prices up to an exploitive level that ultimately causes trade imbalances, economic downturns, and wars.
This nonsense about anti-trust and "evil monopolies" is a bunch of liberal lie nonsense put forward by Progressives in the early 20th century.
Quite the contrary, the monopolies that came out of the industrial revolution and the artificially high prices they extracted from the market as well as the unnaturally low wages they paid their workers are the reason that the socialist policies of the twentieth century became popular enough that they could be passed into law and inflicted on our nation and the rest of the world. Although this historical fact conflicts with the theoretical libertarian ideal, monopolies create popular support for socialism and other leftist ideologies that oppress people and give greater control to the leftist elite. It is no coincidence that almost every monopoly is controlled by a liberal Democrat. Minimal regulation that keeps the market open, safe, and competitive creates the best long-term outcome for businesses, workers, consumers, and a healthy overall economy.