The answer to 'my shut-off valve smells bad' is that a foul odor coming from a shut-off valve is usually caused by a buildup of debris, mineral deposits, or bacterial growth. This can happen over time as water sits stagnant in the valve.
Some common symptoms include a rotten egg or sewage-like smell, discoloration around the valve, and potentially a leak if the valve is corroded. To fix this, you'll want to turn off the water supply, disassemble the valve, and thoroughly clean it to remove any buildup. If the valve is severely damaged, it may need to be replaced by a professional plumber.