Thanks, this is a very clear article on the most important ingredients of HFT. I am curious to hear the apology but I am not sure there is anything to apologize for. The market marker is taking on substantial risk of adverse selection and is being compensated for it. As long as legal means are used to achieve that end, this is not really different from any other business strategy. Causal implied benefits to society from any business are subjective and the most important consequences are usually latent.

Anyways, I am curious to know if elaborate prediction models are used here or it is mostly a game of adjusting prices based on correlated instruments? Also, how does one build a simulator for such strategies?