The first answer is correct when it says that monopolies are bad, but it does not correctly set out the economic criticisms of a monopoly.
First, it is not correct to say that monopolies can simply raise prices and that customers will be "forced" to pay what the monopoly charges. A monopolist faces an downward sloping demand curve, not a vertical one. If a monopolist raises its prices, it will lose customers. Therefore, monopolists cannot simply raise prices as high as they like. Customers may choose to buy or to go without and if the prices are too high, they will not buy.
Second, the first answer does not really set out the main economic reason for opposing monopolies. Monopolies are a bad thing (to economists) because they misallocate resources. Resources are properly allocated when the price of the last unit produced (marginal cost) is equal price of that last unit. In perfect competition, firms produce at the point where MC = Price and resources are efficiently allocated. By contrast, a monopolist produces a quantity where MC < Price. This means (this is most easily seen on a graph) that the quantity produced will be lower than equilibrium and that the price charged will be higher than equilibrium. This is a misallocation of resources.
So, the first answer is correct to say that monopolies are a bad thing, but it does not explain why in proper economic terms.