Let's say there's a runaway superintelligence equipped with an optimization function that says it should produce a maximal amount of paperclips. Then either its objective, "produce a maximal amount of paperclips" is defined literally or not.
If it's defined literally, e.g. "make your sensors return this data consistent with lots of paperclips having been produced", then it's far easier for the AI to corrupt its sensors with paperclip porn than actually destroy the Earth, and there's no problem.
On the other hand, if the objective is not defined literally, then the AI must be able to understand fuzzy instructions in the way the humans intended it to. In that case, it would be no problem to tell the AI to not be an ass either.
So the problem happens when the AI isn't intelligent enough, and uses heuristics instead of properly optimizing. The extreme case would be grey goo, which doesn't think at all, but just blindly consumes everything. Giving extreme power to something with limited intelligence is generally a bad move.