1 - buy up all existing shares at P
2 - probably buy up all the shares at P+dP as well (because all US markets have price-time priority)
3 - place sell orders at P+dP that would fill this guy's X contracts
4 - According to your story, they (or someone else) then try selling at price P again.
This means they: 1 - Took a large long position
2 - Took an even larger long position at a worse price, paying commission for this
3 - Managed to sell some of their position at the same price as (2)
4 - Are now trying to sell back their position at P.
If you look through these steps, you just end up losing money, while taking on unnecessary market risk.
It would be great if you included any sort of evidence or argument.
Reading on to the other comments, it looks like you're throwing out a lot of accusations and claims. I don't know what you think you know, but from the looks of it, you don't really know HRT's business. I don't really these days, but I knew it years ago, and it's not from taking client money or arbitrage or some weird scam. It's not magic but the world of algo trading isn't a ponzi scheme.