Hi dear recipient, i trust that all is fine. Here is what I got to say in reagrds to your query, it is very important on first hand to differentiate to type of Brokers, There are Straight To Processing Brokers (STP) also known as ECN(Electronic Communication Network) and finally Market Makers, For the first to your Order will be directly placed in the intrabank sphere, and the broker will earn the spread by providing your quote the best price he has from surveying all the liquidity providers, but slippage and requote can occure, and ECN/STP may not fill your orders because of lack of liquidity, whereas the Market Maker brokers provide your with the best spread or commisions that suits them, they are dealing desk brokers, you trade with them, your orders do not necessarily go through processing and there may be conflict of interest when it comes to the price but that does not mean that a market maker will profit from you losses, as a market maker they are also exposed to the fluctuation of the market that can result into losses from their side, but every counter measures are taken keep them safe, and continue to provide you with liquidity.
I hope i have helped you. But everything comes down to you as a trader in regards of your performances and ability to trade. some will prefer STP/ECN, others will prefer Market Makers.