The fear I have isn't an AI doing things by itself, but being good enough so that if Joe Evil gets his hands on the AI, he can single-handedly (with AI help) break into secure databases, or something.
You know how a lot of us on HN talk about how security is just a latent concern for companies, but luckily there aren't enough hackers to take advantage of the massive number of bugs in every bit of code ever written? Well, a future powerful coding AI running on second-hand Etherium mining rigs in some extremist's basement in Chicago can probably do a lot more damage than a handful of state sponsored hackers in Russian and North Korea!
