Market makers are also called desk dealing brokers (DD). Market makers go against their clients i.e. to the opposite side of the trade to create a market for the client.
This may cause a conflict of interest since the broker may position themselves to acquire higher profits. This will not be good for the client. Not all DD brokers will do this, but there is a good possibility for this to happen.
Therefore, it is always clever to choose an NDD broker, preferably an STP broker to avoid this conflict of interest and to make sure your brokers main aim is to create profits for you and not them.