ELI5: why exactly did the clients get bad execution? Does sending order flow to trading firms in itself cause unfavorable execution? Or did the trading firms treat order flow from RH differently than from other sources? Or is it something else? And does the fact that RH was receiving large payments for order flow impact the quality of execution?
Similarly in any given market there's the ratio of informed trading to uninformed trading. Generally the former are large institutions, and the latter are small retail daytraders like the Robinhood customers. Since active management is a zero sum game, a venue with all informed traders is not a place you want to trade at.
By law the public exchanges are not allowed to segment order flow. Imagine a club that's legally prohibited from discriminating based on gender at the door. As you can imagine, the ratio at these venue is extremely biased towards informed traders.
Dark pools and internalizers are not bound by that restriction. However they're legally obligated to meet or beat the best price in the public market. (More on that in a second.) Retail brokers like Robinhood are essentially like club promoters, whose job is to get paid for bringing as many pretty girls to these venues as possible. And on the other side the informed traders at these venues (usually just the internal prop desk) pay big money for the privilege of trading somewhere with such a good ratio.
Is this a bad deal for the girls being herded by club promoters? To a first order approximation: no. At the very least they're getting free entry and drinks, instead of paying listed fees at the public venues. Sometimes they even get other perks like free meals or zero-commissions from the promoter. Still it feels exploitive because the promoter is making far more money off the girls than any fringe benefits they're getting.
As a second-order effect, the segmentation may degrade the overall ecosystem. Eventually all the girls wind up at the private clubs, and the public ones become ~100% dudes. Analogously the public lit exchanges have become highly toxic. Which is bad because market makers set prices based on the quality of the flow they interact with.
Remember that price protection on dark pools is based on the best price available at the lit exchanges. This creates a negative feedback loop. Dark venues can improve on prices at lit exchanges by segmenting order flow. Which forces lit market makers to worsen their prices. Which gives the dark venues even more of an advantage, allowing them to price out even more desirable order flow. Which then makes the lit order flow even more toxic, and the public quotes worsen. Which then drives even more flow to dark pools and internalizers...