The main scenario where HFT firms increase volatility is when their algorithms break (either due to a programming error, or because they cease to accurately model the market due to a change in market conditions). In that case, the misfiring algorithm will increase the market volatility, and lose the HFT firm money.

You absolutely right that firms without co-located data centers are at a disadvantage. Thankfully, this only really matters if you are trying to compete with HFT firms. So it's not a big issue for people who trade on a longer time scales. The HFT firms do make some money off of these long term investors, and in exchange provide increased liquidity, decreased spreads, and decreased volatility. (if the HFT firms weren't offering the best prices for the stocks, then the long term investors wouldn't be buying from them)