I'm not an expert here by any means but I think HFT seems like stealing to most of us because it is largely hidden from public view and from any serious scrutiny. The price it extracts is largely unknown and not well understood by those who are paying it. The biggest HFTs seems like a shadow government running the markets and extracting a hidden trade tax.
As for the argument that the value they provide is "liquidity", that also seems dubious. I've seen that argument thrown around a few times now, but I'm not entirely convinced that HFTs, as they exist and work now, are achieving this goal fairly, nor that this is necessarily the best solution.
Do you think that pre-HFT trading practices are transparent and easily understood, and that they're under a proper amount of scrutiny? Are large banks trading at lower frequencies less shadow governmenty? I posit they are not, and I don't understand why the speed of the transactions would be dispositive.
(I'm told by knowledgeable people that there are indeed some shady things that happen in HFT, such as quote stuffing, that ought to be fixed, which makes sense. But I don't think that brings us anywhere near "shadow government", nor does it seem like a damning problem for HFT as a concept.)
The odd-eighth spread pricing scandal was also something that was hidden from public view for decades, and it had a far more profound impact on the fairness of the markets!
Unless you can say that the "price extracted" is fair and self regulating without any clear and transparent oversight, I think there is probably room for improvement and a benefit from increased scrutiny. However, as I said, my knowledge of the internal workings of the markets is still pretty limited.
If you understand the scandal, you understand the answer to that question: automated trading, opaque as it is, strangled the spreads that human market makers had been capitalizing on.
As for the argument that the value they provide is "liquidity", that also seems dubious. I've seen that argument thrown around a few times now, but I'm not entirely convinced that HFTs, as they exist and work now, are achieving this goal fairly, nor that this is necessarily the best solution.