I looked into market maker stratagy and they don't always trade against the trader. Firstly they balance trader positions against each other - so if ten traders sell euros they look for ten traders who are buying, they get their cut off the spread, no problem. When there are orders that cannot be paired they hedge by either trading with or against the trader according to how they see the market. Whoever is on the desk has to be a real hot trader, because if they get it wrong the company get a margin call themselves. This is probably why some MMs have to manipulate prices, make endless requotes, or just turn off the server. Marketing making is high risk.
Of course seom are just downright dishonest and may trade 'naked', never even passing your order on and in this case then if you lose they get the lot.
However, ECNs may be dinhonest as well, though ECN is much less risky so they should not need to. In the end though you have to check the individual companies record carefully, which is why the FPA is so important!
