While some people are drawing lessons from this incident about HFT, I've seen no indication Knight was a major player in that, the summary says they were "*engaging in market making, electronic execution, and institutional sales and trading." I.e. those actions are on the behalf of others, plus stock exchange market makers have obligations to keep what they're responsible for liquid.
It depends on what your definition of HFT is. But they were frequently responsible for ~20% of equity volume in a day. They were market making purely electronically & at low latency. That hits nearly every definition of HFT that I know.
