This is missing the context that Robinhood is required to seek the "best" execution of trades [1]. So Citadel is making a profit yes, but the customer is not necessarily harmed in that profit as Citadel may still be delivering a better execution than other market makers or the actual exchange itself.
[1] https://www.sec.gov/fast-answers/answersbestexhtm.html
Robinhood publishes their execution numbers (as required by the SEC), along with other brokers. At a quick glance, there is nothing out of the ordinary. It's also worth noting that you cannot directly compare numbers as different platforms have different trading behavior.
https://robinhood.com/us/en/about-us/our-execution-quality/