Just a guess, but sounds like the owner couldn’t sell the house so wanted to collect insurance.
Turned the gas on thinking it would only destroy the one house.
Owner was gone for a few days and figured it would just be a fire. The leak would need to be before the furnace and the furnace shut down (to prevent the electric pilot from igniting the gas). The gas built up until it ignited from another source on a higher floor. Just speculation, but I agree with the insurance angle
As I commented before, more than likely they got their entire view of reality based on what they have seen in the movies and on TV.
No concept of what really happens, just the movie version.