The public markets are a series of zero-sum games. When you fully understand this, it becomes apparent that almost any profit-making strategy for engaging with them can be described as a form of "rigging". Every dollar Virtu makes has to come out of someone else's pockets.
The important thing to know is "whose pocket".
People on HN love to play six-degrees-of-Mom's-retirement about this issue. That game doesn't actually work. Consider two scenarios:
(a) By increasing the cost for billion-dollar broker/dealers to shop blocks, electronic trading systems are making it more expensive for mutual funds to trade and thus extracting rents from the hides of pension funds; the pockets picked are ultimately those of ordinary Americans.
(b) By preying on naive market-making strategies, electronic trading systems are disrupting an entrenched cartel of billion-dollar market players, and in the process driving trading spreads to historic lows without measurably impacting volatility in the market; the pockets picked are those of other, more brazen pickpockets.
The difference between (a) and (b) is crucially important. If the status quo ante is bad, and the new situation is better, then organizing against that change is a positive action taken in favor of unfairness.
Significant evidence points to (b).