I can give an example of how they are supported by the state. For example, in some countries, the interest rate given to banks is lowered under the name of "policy rate". Even though the interest rate is lowered, the banks give loans to the public at a higher interest rate. They both buy the money they lend cheaply and sell it to the public expensively. This is not a positive side of the banks. In this way they are making the people poorer.
This wouldn't be an a big issue if banks weren't rescued when they go under. Obviously the large banks get preferential treatment with bailouts and special rates. The banks sustain the country's currencies so there's an interest by the state to cover them if they fail. This type of relationship is dangerous for the reason that currency no longer becomes zero sum, and there's incentive by the state to generate new currency should the banking industry need it. Also causes reckless lending behavior if the banks know if there losses are covered.