Pure capitalism is letting the market decide which leads to the monopolization of industries.
Nobody's ever succeeded in establishing a coercive monopoly without government backing. In a free market, monopoly is a non-issue. For example, when Alcoa was the only vendor of Aluminum in the United States, the pricing of aluminum fell continuously.
Standard Oil and Bell says otherwise, in different ways.
The first one is just a matter of "are you big enough, ruthless enough and no rules stop you, you can get rid of competition that way".
The second one - Bell - is interesting. For some services, like telephony, if you don't have government regulation you will get a natural monopoly. The phone companies would earn more money if they merged - no need to ever compete on price, or duplicate infrastructure. The price would be based on the value to consumers, not on the marginal cost of providing it as in a perfect market. And competition would be hard to come by - refuse to receive and make calls to this network. Knowing this, a competetive network would never appear in the first place.