Renaissance Technologies avoided $6bn in taxes by using derivatives to pretend their ultra-high frequency trading profits were actually long-term capital gains.
http://www.bloomberg.com/news/2014-07-21/renaissance-avoided...
Renaissance is one of the biggest and most successful hedge funds of all time.
SAC was another of the most successful hedge funds of all time - government couldn't get anyone to prove insider trading directly implicating the founder, but they got enough people around him to confess that they've basically cut the fund down to a personal funds only shop - Point72.
What else ...
Plenty of socially questionable legal strategies in HF land. Big one is stock buybacks - rational responses to Federal Reserve interest policies. If debt is priced too cheaply, HF will push companies to lever up to buyback stock. This is a socially useless form of activity, which increases business risk based on capital structure theory. Short term payoff, the debt will never go away.
Returns - the Hedge Fund industry in aggregate is so far off the S&P500 over the past 5 years that perhaps it ought not to exist?
http://www.bloomberg.com/news/2014-09-17/hedge-fund-performa...
So - to deliver middling average returns, the hedge fund industry will charge 4x to 20x the cost of an index fund.
Much of the money for the funds comes from public pension funds.
We can go on...