Destroying all civilization doesn't mean destroying all humans, but just the structures that hold society together. A runaway heating scenario may be a pretty effective way of doing that - all crops die, wars for food, nukes get used.
I think prottero worked on a method of synthesizing sugar at much lower cost via bacteria, so carbs may be possible too. Combine that with how really rich humanity is, so i think a technical solution would be availbe to solve those food issues, and what better incentive than a potential nuclear war, to convince people to collaborate on that ?
