An AI doesn't have to understand the world in order to destroy it. It needs to be hooked up to the one technology which could credibly destroy the world: a nuclear weapons system. A very poorly designed nuclear weapons system that doesn't have any meaningful interlocks.
Myself, I'm going to remain much more worried about the natural intelligences which have been hooked up to nuclear weapons systems.
It doesn't need to be something big and scary like a nuclear weapons system.
It could be hooked up to the stock market, and it could make entirely rational decisions based on its objective (profitable trades) and these actions can result in imbalances leading to famine in certain regions, increased pollution, unsustainable depletion of natural resources etc.
We are already hooked up. The AIs are just amplifications of our own narrowly focused objectives.
Algorithmic trading systems have already gone haywire, repeatedly in fact. All major stock markets "circuit breaker" systems in place to halt trading in the event of extreme volatility for specifically this reason.
The sort of long range damaging activities you mention are unlikely though, as algorithmic trading systems in general take their long term cues from humans.
Myself, I'm going to remain much more worried about the natural intelligences which have been hooked up to nuclear weapons systems.