Here's an article on brokers trading against you if you are interested.
In short, it says the following. Brokers tend to fall into one of the three categories, Market Makers, STP brokers and ECN's.
An STP broker passes all it's clients trades directly to liquidity provider and has nothing to gain from it's clients losing and never trades against them. If anything, an STP broker would want to see it's clients succeed and make money so that they keep on trading.
ECN brokers simple provide a true market place where all their clients, retail traders, banks, financial institutions can trade against each other on an equal basis - ECN brokers don't care which of their clients wins and which of their clients lose as they're clients are effectively trading against each other.
Market Makers (brokers with a dealing desk). These brokers initially take the otherside of their clients trades and in doing so literally make a market and provide liquidity. They could, if they wanted to, hold on to the otherside of their clients trades and profit from their clients losses if they wanted to but they probably don't. Most market makers probably just seek to match their clients trades with the trades of other clients who want to do the opposite and hedge the difference (what they can't match) in the market and seek to profit from the spread.
Hope that helps.