Your spark was arcing where it didn't belong, at the plug gap. You can take a simple 12 volt test light and check for spark leakage. Ground the tester to the cooling fins on the block. Then probe around the plug wire, coil and plug. If you see a spark jump onto the tester probe, you have spark leakage. Again going where it doesn't belong. Start the engine and probe around.
I would de-carbon that cylinder. Might have to soak it and scrub later. Carb cleaner works pretty good. Make sure your allow any liquid/carbon/dirt to drain out of the cylinder while cleaning/spraying carb cleaner. Seafoam and B-12 are good too. Might soak with WD-40 (mainly a solvent) and scrub away. Clean is clean. Think hospital clean.
slomo