HFT doesn't inflate or deflate any valuations. They operate at the market microstructure level and provide liquidity. HFT firms have no impact on the long-term value of assets.

HFTs can siphon away profits from the people actually doing good investing, but Not all HFT is created equal. There have been some pretty high profile instances where HFTs have increased market volatility and caused a "flash crash".

HFTs that trade at the microsecond scale probably aren't valuable to society.