Earlier quoted context omitted.
HFT has significantly lowered trading costs for all investors. Here's a statement from the CEO of Vanguard espousing this position: https://www.cnbc.com/2014/04/25/vanguard-chief-defends-high-... Because of automation, the amount of resources used for trading have actually decreased. Computers are a lot cheaper that the many many humans who used to be involved in the process.
I think it’s more nuanced in reality. There are liquidity providing HFT strategies that profit off the spread - this is good for people moving big size like Vanguard. Then there are liquidity taking strategies that profit by picking off quotes in anticipation of a probable price move. I don’t see how those strategies help Vanguard. Also, most HFT firms do a mix of both types strategies...
Even the taking of liquidity in the anticipation of a price move is not inherently bad. It is only bad if it degrades market efficiency (for example, by adding instability). An aggressive order that anticipates a price move due to an existing market inefficiency (for example, a sudden over-reaction to a report) benefits the market by speeding price discovery and adding stability.
As to your question of how an aggressive HFT strategy could benefit Vanguard, consider that if Vanguard places a passive order, it is because Vanguard hopes to attract an aggressive matching order. If they are unable to attract one within their time requirement, they will likely need to change their order to a less-profitable price. Their time requirement could be quite short during times of great volatility. An HFT strategy able to more-quickly recognize that Vanguard's original price was attractive would save Vanguard the cost of a price change. This benefit can be particularly large during significant market stress when participants are forced to liquidate assets in order to manage risk.