There's Citadel Asset Management, the quant hedge fund (which is probably the one that lent money to Melvin, though I'm not sure about that), and Citadel Securities the HFT / market-making show (they have the same owner).
HFT market-making using retail order flow isn't front-running - the price the customers get is the same regardless ("Best Bid Offer" as brokers are legally required to provide in the US). The difference is, once that the HFT market maker holds the stock, they can get rid of it more easily (i.e. they can close the trade) because (the assumption is that) the retail flow is "noise" (uncorrelated with future price movements) as opposed to professional/institutional traders (e.g. other hedge funds executing their strategies), where a big problem is negative selection (i.e. you're more likely to execute a trade with someone that has better information than you).