I can't tell if this is in jest... if you're actually interested, the role of HFT in markets is hotly debated. The HFT side argues that they improve markets by

I can't tell if this is in jest... if you're actually interested, the role of HFT in markets is hotly debated. The HFT side argues that they improve markets by providing liquidity - and that's true to an extant, spreads have narrowed drastically over time. And yes, there is the opposite side arguing they are purely "parasites", and exchanges like LTSE have been built specifically to counter HFT (Flash Boys, etc.) Perhaps I don't like the word "criminals" being thrown around so loosely.