The graph below shows the relationship between the various costs, and revenues of a monopoly and how they interact with consumer demand.
If you look at the Y axis (where prices and costs are represented), you will see three price levels. The highest is the price the monopoly charges consumers for goods in a monopoly outcome. The middle price is both the price charged to consumers, and the cost paid by the firm in a perfectly competitive outcome. The lowest price is the cost paid by the firm in a monopoly outcome.
If you look at the X axis (where the firm's output is shown), you will see two different quantity levels. The highest level of output is achieved during the competitive market outcome where marginal benefit (shown by the consumer demand) is equal to marginal cost. The lower quantity is achieved when the firm has monopoly power, and restricts output.
So this post has shown graphically why monopolies are bad for consumers. It is because a monopoly can both reduce quantity supplied to the market (meaning less people get what they want), and at the same time charge higher prices for the goods that they do sell. While this may seem obvious for some, economics is the science of explaining observed outcomes with models. And this model definitely shows why what we see happening is occurring.