Why might a monopoly hurt a market

A monopoly can charge higher prices and reduce economic welfare.

A monopoly is able to control all output of a certain good or service, meaning that they are the only option for buying a certain thing. This means that they can charge whatever price they want, forcing people to pay exuberant prices for things they need. This can hurt the consumer part of the market and give monopolies an unfair advantage, harming the buying/selling cycle.