Capitalism is the use of capital a a tool of productivity. All nations, Democratic, Communist, Authoritarian, etc. must use capitalism to get things done. It's called investment, how nations, businesses and families build wealth. What we call capitalism is private ownership. Capital is a powerful tool and can lead to more capital until you get concentrated ownership centers we call Oligarchs and sometimes, corporations. The trouble is that they tend to use the power of their capital for their benefit instead of the benefit of the nation or citizens. Actually, they even tend to compete with the citizens for wealth and that naturally leads to wealth inequality. That is why people thought of socialism, so that the wealth centers weren't competing with the citizens, but instead benefiting them. (Communism is just socialism by revolution, a problem because revolutions cause chaos, an environment best for psychopaths.)

The problem is that capital is a power that tends to concentrate capital and in unregulated capitalism (ownership) that power eventually leads to citizens being cash flows for rentiers, effectively wage slaves.