Yes, the difference between theory and practice can be startling in finance. But I am more wary about fleecing the customer than about counter parties in a high frequency trade. [1] And of course there's also always making money by rent-seeking behaviour. E.g. the implicit subsidy banks get in lower borrowing costs on the market once they are to big to fail.

[1] Mutual fund managers or hedge funds who take a lot of fees are probably quite a drain on your their clients returns.