What can cause the world to end in a fire?

Answer 1

• Runaway volcanism (not likely).
• Massive object impact (not likely).
• The sun swelling to a red giant (most likely).
• Earth going outside the green zone and too close to the Sun (very likely).
• "God" or a deity (not likely).

Nuclear war would not be likely to cause destruction by fire; there is a miniscule chance of setting the atmosphere alight, but nuclear winter (extreme cooling caused by fallout) is more likely in the event of absolute nuclear war.