A fear of mine, that I think wouldn't be too hard (relatively speaking) would be to interrupt bid/ask sizes in markets, since it's largely machines trading now with no humans able to step in like a floor specialist. Effecting the bid/ask sizes could cause a massive over supply of equities and cause it to crater, which could induce more machine selling simply because that's how their algorithms are written. The market could crash 10% before anyone knew what was happening. There are breakers that are built to prevent this but high leverage in ETF securities, etc could exacerbate the event. In effect, causing an '87-like crash from a hack. The '87 crash was also largely machine driven.
What you're describing happened to Ethereum this week [1]. On one exchange (GDAX), a large sell order pushed the price down by 30% or so, which in turn triggered a lot of (market) stop loss orders, pushing the price farther down. In the end, a couple of thousand ETH traded hands for $0.10 while the price at other exchanges was around $300.
I share your fear. At the same time, it's also important to remember that most other instruments trade at way deeper order books, with orders of magnitude greater liquidity and massively better controls (e.g. circuit breakers).
