I'm not so sure its possible to draw a such a line, HFT likes to make itself out to be the great saviour doing everyone a favour, but at its heart, HFT only makes its money one way ... spoofing, queue-jumping and other toxic activity. Afterall, if HFT were only "doing good" and providing liquidity then they would not need to spend millions on specialist fibre and radio links etc. etc. would they !
This is incredibly misguided. How do you explain statistical arbitrage or flat arb? And by virtue of creating efficiency you have to be fast. This is backward thinking. HFT has a bad PR problem due to the loonies (Haim Bodek, Brad Katsuyama, etc) yelling about spoofing and queue hoping but not answering how retail traders can pay 50 mils a share to trade or why there's tight markets in names that were $1 wide a few years ago.
@quant1 I suspect you and I will have to agree to disagree. Given your username "quant1", I suspect you are somewhat extremely biased in favour of your HFT friends !
you are onto a broader question why do people like to grumble when events are in their favor or specifically what benefit do Haim Bodek, Brad Katsuyama, etc, yelling about spoofing and queuing, hope to gain from their howling?